app_voicemail/IMAP: check mailstream not NULL in leave_voicemail

The function leave_voicemail checks if expungeonhangup is set,
but does not check if IMAP stream is closed,
so it could call imap function with NULL stream.
This leads to segfault.

ASTERISK-28505 #close

Change-Id: Ib66c57c1f1ba97774e447b36349198e2626a8d7c

apps/app_voicemail.c

@@ -7179,7 +7179,7 @@ leave_vm_out:
 #ifdef IMAP_STORAGE
 	/* expunge message - use UID Expunge if supported on IMAP server*/
 	ast_debug(3, "*** Checking if we can expunge, expungeonhangup set to %d\n", expungeonhangup);
-	if (expungeonhangup == 1) {
+	if (expungeonhangup == 1 && vms->mailstream != NULL) {
 		ast_mutex_lock(&vms->lock);
 #ifdef HAVE_IMAP_TK2006
 		if (LEVELUIDPLUS (vms->mailstream)) {