kv/kvserver: TestReplicaRangefeedErrors failed [NPE in muxrangefeed]

kv/kvserver.TestReplicaRangefeedErrors [failed](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Ci_TestsAwsLinuxArm64_UnitTests/21117192?buildTab=log) with [artifacts](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Ci_TestsAwsLinuxArm64_UnitTests/21117192?buildTab=artifacts#/) on release-25.4 @ [b9b7a4c61d5ffff4bcf5a54eba0b3e96074975b9](https://github.com/cockroachdb/cockroach/commits/b9b7a4c61d5ffff4bcf5a54eba0b3e96074975b9):

```
| pkg/util/ctxgroup/ctxgroup.go:151
| github.com/cockroachdb/cockroach/pkg/kv/kvclient/kvcoord.muxRangeFeed
| pkg/kv/kvclient/kvcoord/dist_sender_mux_rangefeed.go:93
| github.com/cockroachdb/cockroach/pkg/kv/kvclient/kvcoord.(*DistSender).RangeFeed
| pkg/kv/kvclient/kvcoord/dist_sender_rangefeed.go:207
| github.com/cockroachdb/cockroach/pkg/kv/kvclient/rangefeed.(*dbAdapter).RangeFeed
| pkg/kv/kvclient/rangefeed/db_adapter.go:82
| github.com/cockroachdb/cockroach/pkg/kv/kvclient/rangefeed.(*RangeFeed).run.func1
| pkg/kv/kvclient/rangefeed/rangefeed.go:368
| [...repeated from below...]
Wraps: (2) attached stack trace
-- stack trace:
| runtime.gopanic
| GOROOT/src/runtime/panic.go:791
| runtime.panicmem
| GOROOT/src/runtime/panic.go:262
| runtime.sigpanic
| GOROOT/src/runtime/signal_unix.go:917
| github.com/cockroachdb/cockroach/pkg/kv/kvclient/kvcoord.(*activeMuxRangeFeed).start
| pkg/kv/kvclient/kvcoord/dist_sender_mux_rangefeed.go:284
| github.com/cockroachdb/cockroach/pkg/kv/kvclient/kvcoord.(*rangefeedMuxer).startSingleRangeFeed
| pkg/kv/kvclient/kvcoord/dist_sender_mux_rangefeed.go:224
| github.com/cockroachdb/cockroach/pkg/kv/kvclient/kvcoord.divideSpanOnRangeBoundaries
| pkg/kv/kvclient/kvcoord/dist_sender_rangefeed.go:454
| github.com/cockroachdb/cockroach/pkg/kv/kvclient/kvcoord.divideAllSpansOnRangeBoundaries
| pkg/kv/kvclient/kvcoord/dist_sender_rangefeed.go:234
| github.com/cockroachdb/cockroach/pkg/kv/kvclient/kvcoord.muxRangeFeed.func2
| pkg/kv/kvclient/kvcoord/dist_sender_mux_rangefeed.go:90
| github.com/cockroachdb/cockroach/pkg/kv/kvclient/kvcoord.muxRangeFeed.Group.GoCtx.func3
| pkg/util/ctxgroup/ctxgroup.go:200
| golang.org/x/sync/errgroup.(*Group).Go.func1
| external/org_golang_x_sync/errgroup/errgroup.go:93
| runtime.goexit
| src/runtime/asm_arm64.s:1223
Wraps: (3) runtime error: invalid memory address or nil pointer dereference
Error types: (1) *withstack.withStack (2) *withstack.withStack (3) runtime.errorString

goroutine 95424 [running]:
github.com/cockroachdb/cockroach/pkg/util/ctxgroup.Group.Wait({0x4015cd9c40?, {0x8354278?, 0x40030a0be0?}, 0x401b869e90?})
pkg/util/ctxgroup/ctxgroup.go:151 +0xc8
github.com/cockroachdb/cockroach/pkg/util/ctxgroup.GoAndWait({0x83542b0?, 0x4014dd36e0?}, {0x4006737c60, 0x2, 0x66dc900?})
pkg/util/ctxgroup/ctxgroup.go:224 +0x10c
github.com/cockroachdb/cockroach/pkg/kv/kvclient/rangefeed.(*RangeFeed).run(0x400f777a40, {0x83542b0, 0x4014dd36e0}, {0x83b7560, 0x4025f90f00}, 0x0)
pkg/kv/kvclient/rangefeed/rangefeed.go:394 +0xa04
github.com/cockroachdb/cockroach/pkg/kv/kvclient/rangefeed.(*RangeFeed).start.func1({0x8354278, 0x4025f90f50})
pkg/kv/kvclient/rangefeed/rangefeed.go:254 +0x3a0
github.com/cockroachdb/cockroach/pkg/util/stop.(*Stopper).RunAsyncTaskEx.func1({0x8354278, 0x4025f90f50})
pkg/util/stop/stopper.go:440 +0x88
created by github.com/cockroachdb/cockroach/pkg/util/stop.(*Stopper).RunAsyncTaskEx in goroutine 90426
pkg/util/stop/stopper.go:438 +0x104
```
Help

See also: [How To Investigate a Go Test Failure \(internal\)](https://cockroachlabs.atlassian.net/l/c/HgfXfJgM)

/cc @cockroachdb/kv-triage

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Jira issue: CRDB-60473