acl.c: Coerce a NULL pointer into the empty string

If an ACL is misconfigured in the realtime database (for instance, the
"rule" is blank) and Asterisk attempts to read the ACL, Asterisk will
crash.

ASTERISK-28978 #close

Change-Id: Ic1536c4df856231bfd2da00128f7822224d77610

main/acl.c

@@ -578,7 +578,7 @@ static struct ast_ha *append_ha_core(const char *sense, const char *stuff, struc
 	struct ast_ha *ha;
 	struct ast_ha *prev = NULL;
 	struct ast_ha *ret;
-	char *tmp, *list = ast_strdupa(stuff);
+	char *tmp, *list = ast_strdupa(stuff ?: "");
 	char *address = NULL, *mask = NULL;
 	int addr_is_v4;
 	int allowing = strncasecmp(sense, "p", 1) ? AST_SENSE_DENY : AST_SENSE_ALLOW;