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/os_unix.c b/src/os_unix.c
index 6387a77..d80fb1d 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -5998,6 +5998,11 @@
 		rest -= msec;
 	}
 # endif
+# ifdef FEAT_SOUND_CANBERRA
+	// Invoke any pending sound callbacks.
+	if (has_sound_callback_in_queue())
+	    invoke_sound_callback();
+# endif
 # ifdef FEAT_MOUSE_GPM
 	gpm_process_wanted = 0;
 	avail = RealWaitForChar(read_cmd_fd, msec,