core: Ensure presencestate subtype and message are NULL.

When retrieving presence state information there is no
guarantee that the subtype and message passed in are
set to NULL. This change ensures they are.

ASTERISK-26397 #close

Change-Id: If38cd730e409e9a9b6eb9adef6591d15a9e61f86

main/presencestate.c

@@ -161,6 +161,9 @@ static enum ast_presence_state ast_presence_state_helper(const char *presence_pr
 		[AST_PRESENCE_DND]         = 7
 	};
 
+	*subtype = NULL;
+	*message = NULL;
+
 	while ((label = strsep(&labels, "&"))) {
 		enum ast_presence_state next_state = AST_PRESENCE_INVALID;
 		char *next_subtype = NULL;