Use size_t for return value from fwrite.

src/log.cc

Index: src/log.cc
===================================================================
--- src/log.cc	(revision 5694)
+++ src/log.cc	(working copy)
@@ -1378,8 +1378,8 @@
 void Logger::LowLevelCodeCreateEvent(Code* code, LogMessageBuilder* msg) {
   if (!FLAG_ll_prof || Log::output_code_handle_ == NULL) return;
   int pos = static_cast<int>(ftell(Log::output_code_handle_));
-  int rv = fwrite(code->instruction_start(), 1, code->instruction_size(),
-                  Log::output_code_handle_);
+  size_t rv = fwrite(code->instruction_start(), 1, code->instruction_size(),
+                     Log::output_code_handle_);
   ASSERT(static_cast<size_t>(code->instruction_size()) == rv);
   USE(rv);
   msg->Append(",%d", pos);