Fix --hydrogen-stats.

V8 crashes with this flag when trying to compile hydrogen stubs.

R=jkummerow@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/12045048

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13487 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen.cc

@@ -10196,6 +10196,7 @@ void HTracer::FlushToFile() {
 
 
 void HStatistics::Initialize(CompilationInfo* info) {
+  if (info->shared_info().is_null()) return;
   source_size_ += info->shared_info()->SourceSize();
 }