fixes segfault in func_groupcount

closes issue #16337)
Reported by: Parantido
Patches:
      issue_16337.diff uploaded by dvossel (license 671)
	  Tested by: Parantido, dvossel



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@232268 65c4cc65-6c06-0410-ace0-fbb531ad65f3

funcs/func_groupcount.c

@@ -149,6 +149,10 @@ static int group_function_write(struct ast_channel *chan, char *cmd,
 {
 	char grpcat[256];
 
+	if (!value) {
+		return -1;
+	}
+
 	if (!ast_strlen_zero(data)) {
 		snprintf(grpcat, sizeof(grpcat), "%s@%s", value, data);
 	} else {