fix possible null pointer math

Author: weltling

sapi/cgi/cgi_main.c

@@ -1661,13 +1661,15 @@ PHP_FUNCTION(apache_request_headers) /* {{{ */
 static void add_response_header(sapi_header_struct *h, zval *return_value TSRMLS_DC) /* {{{ */
 {
 	char *s, *p;
-	int  len;
+	int  len = 0;
 	ALLOCA_FLAG(use_heap)
 
 	if (h->header_len > 0) {
 		p = strchr(h->header, ':');
-		len = p - h->header;
-		if (p && (len > 0)) {
+		if (NULL != p) {
+			len = p - h->header;
+		}
+		if (len > 0) {
 			while (len > 0 && (h->header[len-1] == ' ' || h->header[len-1] == '\t')) {
 				len--;
 			}