Crash when attempting to close the same file multiple times in short period

[xusheng6]

Repro steps:

1. Open a large binary
2. Make any changes to the file, e.g., change any function name to "foobar"
3. Hit Command + W multiple times quickly
4. Observe the save dialog appears multiple time asking whether you want to save the file
5. Say "No" to all of them
6. Observe the crash

[plafosse]

Accidental closure

[spoonmilk]

This appears to be caused by a race condition when closing the final tab of a screen. Working toward a fix.

[spoonmilk]

Fixed on dev versions >= 5.1.7888