panic during queue dispatch

# Describe the bug

Just noticed a maddy instance not delivering mail and continuously logging the following error:

```
[...]
Jul 01 20:37:05 maddy[14798]: panic during queue dispatch 6f3f66c4: runtime error: invalid memory address or nil pointer dereference
Jul 01 20:37:05 maddy[14798]: goroutine 2466593 [running]:
Jul 01 20:37:05 maddy[14798]: runtime/debug.Stack()
Jul 01 20:37:05 maddy[14798]: runtime/debug/stack.go:26 +0x5e
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue.(*Queue).dispatch.func1.1()
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue/queue.go:314 +0x85
Jul 01 20:37:05 maddy[14798]: panic({0x1265c80?, 0x1e630c0?})
Jul 01 20:37:05 maddy[14798]: runtime/panic.go:792 +0x132
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/limits/limiters.(*BucketSet).TakeContext(0xc000295b00?, {0x162c2c8, 0xc00631d030}, {0xc005654890?, 0x2?})
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/limits/limiters/bucket.go:152 +0x38
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/limits.(*Group).TakeDest(0xc0003cbd10, {0x162c258?, 0xc0052e2810?}, {0xc005654890, 0xf})
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/limits/limits.go:225 +0x137
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/remote.(*remoteDelivery).connectionForDomain(0xc00620da40, {0x162c258, 0xc0052e2810}, {0xc005654890, 0xf})
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/remote/connect.go:272 +0x68b
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/remote.(*remoteDelivery).AddRcpt(0xc00620da40, {0x162c258, 0xc0052e2810}, {0xc005654888, 0x17}, {{0x0, 0x0, 0x0}, {0x0, 0x0}, ...})
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/remote/remote.go:317 +0x109
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue.(*Queue).deliver(0xc0002f0f20, {0x162bac0, 0x1fd7ac0}, 0xc00296c280, {{0xc00061c300?, 0x13b4ce0?, 0xc00620d860?}, 0xc0052e2240?}, {0x1629c90, 0xc006600858})
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue/queue.go:511 +0x70d
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue.(*Queue).tryDelivery(0xc0002f0f20, {0x162bac0, 0x1fd7ac0}, 0xc00296c280, {{0xc00061c300?, 0xc005bddf30?, 0x41d0c5?}, 0xc0052e2240?}, {0x1629c90, 0xc006600858})
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue/queue.go:389 +0x299
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue.(*Queue).dispatch.func1()
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue/queue.go:342 +0x26a
Jul 01 20:37:05 maddy[14798]: created by github.com/foxcpp/maddy/internal/target/queue.(*Queue).dispatch in goroutine 54
Jul 01 20:37:05 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue/queue.go:302 +0x175
Jul 01 20:37:05 maddy[14798]:
Jul 01 20:37:06 maddy[14798]: panic during queue dispatch 9e3a94f0: runtime error: invalid memory address or nil pointer dereference
Jul 01 20:37:06 maddy[14798]: goroutine 2466860 [running]:
Jul 01 20:37:06 maddy[14798]: runtime/debug.Stack()
Jul 01 20:37:06 maddy[14798]: runtime/debug/stack.go:26 +0x5e
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue.(*Queue).dispatch.func1.1()
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue/queue.go:314 +0x85
Jul 01 20:37:06 maddy[14798]: panic({0x1265c80?, 0x1e630c0?})
Jul 01 20:37:06 maddy[14798]: runtime/panic.go:792 +0x132
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/limits/limiters.(*BucketSet).TakeContext(0xc000295b00?, {0x162c2c8, 0xc0055c33b0}, {0xc001e57727?, 0x2?})
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/limits/limiters/bucket.go:152 +0x38
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/limits.(*Group).TakeDest(0xc0003cbd10, {0x162c258?, 0xc0016fcd50?}, {0xc001e57727, 0x9})
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/limits/limits.go:225 +0x137
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/remote.(*remoteDelivery).connectionForDomain(0xc004fb19e0, {0x162c258, 0xc0016fcd50}, {0xc001e57727, 0x9})
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/remote/connect.go:272 +0x68b
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/remote.(*remoteDelivery).AddRcpt(0xc004fb19e0, {0x162c258, 0xc0016fcd50}, {0xc001e57720, 0x10}, {{0x0, 0x0, 0x0}, {0x0, 0x0}, ...})
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/remote/remote.go:317 +0x109
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue.(*Queue).deliver(0xc0002f0f20, {0x162bac0, 0x1fd7ac0}, 0xc005d621e0, {{0xc002c04300?, 0x13b4ce0?, 0xc004fb1800?}, 0xc004def080?}, {0x1629c90, 0xc003f43788})
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue/queue.go:511 +0x70d
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue.(*Queue).tryDelivery(0xc0002f0f20, {0x162bac0, 0x1fd7ac0}, 0xc005d621e0, {{0xc002c04300?, 0xc005792730?, 0x41d0c5?}, 0xc004def080?}, {0x1629c90, 0xc003f43788})
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue/queue.go:389 +0x299
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue.(*Queue).dispatch.func1()
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue/queue.go:342 +0x26a
Jul 01 20:37:06 maddy[14798]: created by github.com/foxcpp/maddy/internal/target/queue.(*Queue).dispatch in goroutine 54
Jul 01 20:37:06 maddy[14798]: github.com/foxcpp/maddy/internal/target/queue/queue.go:302 +0x175
[...]
```

Not sure what caused it, the instance has been up and working for several days, and there have been no recent configuration changes.

# Steps to reproduce

Not sure yet.

# Environment information

* maddy version: v0.9.5-0.20260516125940-e42741c75777 linux/amd64 go1.24.4