Double click to maximise command prompt window causes crash (Windows, cross-term)

# Bug Report

Using the "double click to maximise window" feature on the Windows command prompt causes the program to crash. Following the stack trace, it appears to be the unwrap on one of the mouse events in the map_key function in backend/crossterm.rs. See the stack trace below.

```
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\src\libcore\macros\mod.rs:15:40
stack backtrace:
0: backtrace::backtrace::trace_unsynchronized
at C:\Users\VssAdministrator\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.40\src\backtrace\mod.rs:66
1: std::sys_common::backtrace::_print_fmt
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\sys_common\backtrace.rs:84
2: std::sys_common::backtrace::_print::{{impl}}::fmt
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\sys_common\backtrace.rs:61
3: core::fmt::write
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libcore\fmt\mod.rs:1025
4: std::io::Write::write_fmt
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\io\mod.rs:1426
5: std::sys_common::backtrace::_print
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\sys_common\backtrace.rs:65
6: std::sys_common::backtrace::print
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\sys_common\backtrace.rs:50
7: std::panicking::default_hook::{{closure}}
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\panicking.rs:193
8: std::panicking::default_hook
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\panicking.rs:210
9: std::panicking::rust_panic_with_hook
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\panicking.rs:471
10: std::panicking::begin_panic_handler
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\panicking.rs:375
11: core::panicking::panic_fmt
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libcore\panicking.rs:84
12: core::panicking::panic
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libcore\panicking.rs:51
13: core::option::Option::unwrap
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\src\libcore\macros\mod.rs:15
14: cursive::backend::crossterm::Backend::map_key
at C:\Users\redacted\.cargo\registry\src\github.com-1ecc6299db9ec823\cursive-0.14.0\src\backend\crossterm.rs:266
15: cursive::backend::crossterm::{{impl}}::poll_event
at C:\Users\redacted\.cargo\registry\src\github.com-1ecc6299db9ec823\cursive-0.14.0\src\backend\crossterm.rs:294
16: cursive::cursive::Cursive::process_events
at C:\Users\redacted\.cargo\registry\src\github.com-1ecc6299db9ec823\cursive-0.14.0\src\cursive.rs:922
17: cursive::cursive::Cursive::step
at C:\Users\redacted\.cargo\registry\src\github.com-1ecc6299db9ec823\cursive-0.14.0\src\cursive.rs:897
18: cursive::cursive::Cursive::run
at C:\Users\redacted\.cargo\registry\src\github.com-1ecc6299db9ec823\cursive-0.14.0\src\cursive.rs:883
19: curse::main
at .\src\main.rs:53
20: std::rt::lang_start::{{closure}}<()>
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\src\libstd\rt.rs:67
21: std::rt::lang_start_internal::{{closure}}
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\rt.rs:52
22: std::panicking::try::do_call
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\panicking.rs:292
23: panic_unwind::__rust_maybe_catch_panic
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libpanic_unwind\lib.rs:78
24: std::panicking::try
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\panicking.rs:270
25: std::panic::catch_unwind
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\panic.rs:394
26: std::rt::lang_start_internal
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\/src\libstd\rt.rs:51
27: std::rt::lang_start<()>
at /rustc/5e1a799842ba6ed4a57e91f7ab9435947482f7d8\src\libstd\rt.rs:67
28: main
29: invoke_main
at d:\agent\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:78
30: __scrt_common_main_seh
at d:\agent\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288
31: BaseThreadInitThunk
32: RtlUserThreadStart
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: process didn't exit successfully: `target\debug\curse.exe` (exit code: 101)
```

# Environment

* Operating system: Windows
* Backend used: cross-term
* Current locale:
```
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_ALL=
```
* Cursive version: 0.14