Fix crash in ControllerEngine::gracefulShutdown()

[uklotzde]

This fixes crash caused by invalid iterator usage when shutting down Mixxx while a controller is connected.

But afterwards Mixxx still crashes when exiting the Controller thread:

Debug [Controller]: Deleting USB Bulk devices...
Debug [Controller]: Deleting HID devices...
[Thread 0x7ffec3fff700 (LWP 22012) exited]
malloc_consolidate(): invalid chunk size

Thread 34 "Controller" received signal SIGABRT, Aborted.
[Switching to Thread 0x7ffee2ffd700 (LWP 22006)]
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50	  return ret;
(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007ffff3623895 in __GI_abort () at abort.c:79
#2  0x00007ffff367e8c7 in __libc_message (action=action@entry=do_abort, fmt=fmt@entry=0x7ffff378fe1d "%s\n") at ../sysdeps/posix/libc_fatal.c:155
#3  0x00007ffff3685dec in malloc_printerr (str=str@entry=0x7ffff37920a0 "malloc_consolidate(): invalid chunk size") at malloc.c:5347
#4  0x00007ffff3686cb8 in malloc_consolidate (av=av@entry=0x7ffff37c19e0 <main_arena>) at malloc.c:4477
#5  0x00007ffff3687460 in _int_free (av=0x7ffff37c19e0 <main_arena>, p=0x17057f0, have_lock=<optimized out>) at malloc.c:4400
#6  0x00007ffff63fb5bb in magazine_cache_push_magazine () at /lib64/libglib-2.0.so.0
#7  0x00007ffff63fb6cf in private_thread_memory_cleanup () at /lib64/libglib-2.0.so.0
#8  0x00007ffff37d1251 in __nptl_deallocate_tsd () at pthread_create.c:301
#9  __nptl_deallocate_tsd () at pthread_create.c:250
#10 0x00007ffff37d1445 in start_thread (arg=<optimized out>) at pthread_create.c:488
#11 0x00007ffff36ff9d3 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

[daschuer]

Thank you for the fix.

Only one minor comment.

[Be-ing]

Is this a regression in 2.3? Does it also apply to the 2.2 branch? I have not noticed this bug.

[Be-ing]

I cannot reproduce this bug, so I am removing the "beta blocker" tag. Of course we should still fix this promptly, but I don't think there's a need to keep pushing back the beta even further.

[uklotzde]

The iteration was plain wrong already in 2.2. Not sure when this undefined behavior actually caused an error. It might depend on both the Qt and compiler version.

[uklotzde]

Iterators become dangling after removing/erasing items:
https://doc.qt.io/qt-5/qhash-iterator.html

[daschuer]

LGTM, waiting for CI. Thank you.