prevent use-after-free during DRM backend shutdown

[andresilva]

The DRM backend crashes during shutdown with heap corruption and null pointer dereferences. Two different stack traces below:

Stack trace of thread 3160:
#0  0x00007f9287e9caac __pthread_kill_implementation (libc.so.6 + 0x9caac)
#1  0x00007f9287e4190e raise (libc.so.6 + 0x4190e)
#2  0x00007f9287e28942 abort (libc.so.6 + 0x28942)
#3  0x00007f9287e299c3 __libc_message_impl.cold (libc.so.6 + 0x299c3)
#4  0x00007f9287ea7565 malloc_printerr (libc.so.6 + 0xa7565)
#5  0x00007f9287ea817c malloc_consolidate (libc.so.6 + 0xa817c)
#6  0x00007f9287ea9460 _int_free_maybe_consolidate.part.0 (libc.so.6 + 0xa9460)
#7  0x00007f9287ea9b12 _int_free (libc.so.6 + 0xa9b12)
#8  0x00007f9287eac3ee free (libc.so.6 + 0xac3ee)
#9  0x00007f9288e5d51a _ZN9Hyprutils6Memory14CSharedPointerIN10Aquamarine12COutputStateEE7_deleteEPv (libaquamarine.so.8 + 0x9051a)
#10 0x00007f9288eb977d _ZN10Aquamarine7IOutputD1Ev (libaquamarine.so.8 + 0xec77d)
#11 0x00007f9288e97ac9 _ZN9Hyprutils6Memory14CSharedPointerIN10Aquamarine10CDRMOutputEE7_deleteEPv (libaquamarine.so.8 + 0xcaac9)
#12 0x00007f9288e8ba7a _ZN10Aquamarine13SDRMConnector10disconnectEv (libaquamarine.so.8 + 0xbea7a)
#13 0x00007f9288e8bb4d _ZN10Aquamarine11CDRMBackendD2Ev (libaquamarine.so.8 + 0xbeb4d)

Stack trace of thread 39495:
#0  0x0000000000000000 n/a (n/a + 0x0)
#1  0x00007ff11332552e _ZN9Hyprutils6Memory5Impl_4implIN10Aquamarine10CDRMOutputEE7destroyEv (libaquamarine.so.8 + 0xf052e)
#2  0x00007ff1133173c9 _ZN10Aquamarine13SDRMConnector10disconnectEv (libaquamarine.so.8 + 0xe23c9)
#3  0x00007ff113317460 _ZN10Aquamarine11CDRMBackendD2Ev (libaquamarine.so.8 + 0xe2460)

Two separate but related issues in the destruction chain:

1. Double-emission of destroy event, first in SDRMConnector::disconnect() and after in IOutput::~IOutput()
2. Unchecked weak pointer dereference in CDRMOutput::~CDRMOutput(). During shutdown, the backend may already be destroyed.

[andresilva]

Updated.