Fix -Wsign-compare warning

Commit 2a0d106 ("http_parser: revert `memchr()` optimization")
introduced a -Wsign-compare warning that manifests itself with
gcc 7.4.0 but not older versions.

The type of p - q is signed when p and q are pointers and was
compared against an unsigned type. Its actual value is always >= 0
however and can be safely cast to size_t.

Fixes: nodejs#471
PR-URL: nodejs#472
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>

http_parser.c

@@ -1497,9 +1497,10 @@ size_t http_parser_execute (http_parser *parser,
           switch (h_state) {
             case h_general:
               {
-                const char* limit = p + MIN(data + len - p, max_header_size);
+                size_t left = data + len - p;
+                const char* pe = p + MIN(left, max_header_size);
 
-                for (; p != limit; p++) {
+                for (; p != pe; p++) {
                   ch = *p;
                   if (ch == CR || ch == LF) {
                     --p;