[wasm] Fix -Wsign-compare warnings.

test/fuzzer/fuzzer.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 
 extern "C" int LLVMFuzzerInitialize(int* argc, char*** argv);
@@ skipping 24 matching lines @@
   uint8_t* data = reinterpret_cast<uint8_t*>(malloc(size));
   if (!data) {
     fclose(input);
     fprintf(stderr, "Failed to allocate %ld bytes\n", size);
     return 1;
   }
 
   size_t bytes_read = fread(data, 1, size, input);
   fclose(input);
 
-  if (bytes_read != size) {
+  if (bytes_read != static_cast<size_t>(size)) {
     free(data);
     fprintf(stderr, "Failed to read %s\n", argv[1]);
     return 1;
   }
 
   int result = LLVMFuzzerTestOneInput(data, size);
 
   free(data);
 
   return result;
 }