Bug#900002: Patch for missing error check in mutter

[Phil Miller]

Control: reassign -1 libmutter-2-0
Control: tags -1 patch upstream

I followed this through a backtrace from a coredump obtained by
systemd-coredump:

      Message: Process 1426 (gnome-shell) of user 112 dumped core.

                Stack trace of thread 1426:
                #0  0x00007fa82bfe346d meta_gpu_kms_new (libmutter-2.so.0)
                #1  0x00007fa82bfe4d0a
meta_monitor_manager_kms_initable_init (libmutter-2.so.0)
                #2  0x00007fa82d9ed557 g_initable_new_valist
(libgio-2.0.so.0)
                #3  0x00007fa82d9ed609 g_initable_new (libgio-2.0.so.0)
                #4  0x00007fa82bf438c1 meta_backend_create_monitor_manager
(libmutter-2.so.0)
                #5  0x00007fa82bf442e4 meta_init_backend (libmutter-2.so.0)
                #6  0x00007fa82bf8be7f meta_init (libmutter-2.so.0)
                #7  0x000056451b90b59a main (gnome-shell)
                #8  0x00007fa82bd27b17 __libc_start_main (libc.so.6)
                #9  0x000056451b90b9ca _start (gnome-shell)

There was a missing check for NULL return on error, which was subsequently
dereferenced, leading to the crash. The attached patch fixed the crash for
me.
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