mod_httapi: Fix Cache-Control max-age= parameter string (length) check

This is probably what you had in mind...
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

src/mod/applications/mod_httapi/mod_httapi.c

@@ -1949,7 +1949,7 @@ static switch_status_t write_meta_file(http_file_context_t *context, const char
 		if (headers && (cc = switch_event_get_header(headers, "Cache-Control")) && (p = switch_stristr("max-age=", cc))) {
 			p += 8;
 			
-			if (p) {
+			if (!zstr(p)) {
 				ttl = atoi(p);
 				if (ttl < 0) ttl = globals.cache_ttl;
 			}