patch 8.1.1851: crash when sound_playfile() callback plays sound

Problem:    Crash when sound_playfile() callback plays sound.
Solution:   Invoke callback later from event loop.
diff --git a/src/misc2.c b/src/misc2.c
index eb6cf55..463b37b 100644
--- a/src/misc2.c
+++ b/src/misc2.c
@@ -4486,6 +4486,10 @@
 # ifdef FEAT_TERMINAL
 	free_unused_terminals();
 # endif
+# ifdef FEAT_SOUND_CANBERRA
+	if (has_sound_callback_in_queue())
+	    invoke_sound_callback();
+# endif
 	break;
     }