fix content-length and chunk-size overflow test

The overflow check didn't work for all possible inputs.

http_parser.c

@@ -1509,8 +1509,8 @@ size_t http_parser_execute (http_parser *parser,
             t *= 10;
             t += ch - '0';
 
-            /* Overflow? */
-            if (t < parser->content_length || t == ULLONG_MAX) {
+            /* Overflow? Test against a conservative limit for simplicity. */
+            if ((ULLONG_MAX - 10) / 10 < parser->content_length) {
               SET_ERRNO(HPE_INVALID_CONTENT_LENGTH);
               goto error;
             }
@@ -1782,8 +1782,8 @@ size_t http_parser_execute (http_parser *parser,
         t *= 16;
         t += unhex_val;
 
-        /* Overflow? */
-        if (t < parser->content_length || t == ULLONG_MAX) {
+        /* Overflow? Test against a conservative limit for simplicity. */
+        if ((ULLONG_MAX - 16) / 16 < parser->content_length) {
           SET_ERRNO(HPE_INVALID_CONTENT_LENGTH);
           goto error;
         }

test.c

@@ -2938,7 +2938,7 @@ test_header_content_length_overflow_error (void)
   "HTTP/1.1 200 OK\r\n"                                                       \
   "Content-Length: " #size "\r\n"                                             \
   "\r\n"
-  const char a[] = X(18446744073709551614); /* 2^64-2 */
+  const char a[] = X(1844674407370955160);  /* 2^64 / 10 - 1 */
   const char b[] = X(18446744073709551615); /* 2^64-1 */
   const char c[] = X(18446744073709551616); /* 2^64   */
 #undef X
@@ -2956,7 +2956,7 @@ test_chunk_content_length_overflow_error (void)
     "\r\n"                                                                    \
     #size "\r\n"                                                              \
     "..."
-  const char a[] = X(FFFFFFFFFFFFFFFE);  /* 2^64-2 */
+  const char a[] = X(FFFFFFFFFFFFFFE);   /* 2^64 / 16 - 1 */
   const char b[] = X(FFFFFFFFFFFFFFFF);  /* 2^64-1 */
   const char c[] = X(10000000000000000); /* 2^64   */
 #undef X