[serializer] small fixes for blink snapshot.

src/snapshot/serializer.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 "src/snapshot/serializer.h"
 
 #include "src/macro-assembler.h"
 #include "src/snapshot/natives.h"
 
 namespace v8 {
@@ skipping 605 matching lines @@
   bytes_processed_so_far_ += kPointerSize;
 }
 
 void Serializer::ObjectSerializer::VisitExternalReference(RelocInfo* rinfo) {
   int skip = OutputRawData(rinfo->target_address_address(),
                            kCanReturnSkipInsteadOfSkipping);
   HowToCode how_to_code = rinfo->IsCodedSpecially() ? kFromCode : kPlain;
   sink_->Put(kExternalReference + how_to_code + kStartOfObject, "ExternalRef");
   sink_->PutInt(skip, "SkipB4ExternalRef");
   Address target = rinfo->target_external_reference();
+  DCHECK_NOT_NULL(target);  // Code does not reference null.
   sink_->PutInt(serializer_->EncodeExternalReference(target), "reference id");
   bytes_processed_so_far_ += rinfo->target_address_size();
 }
 
 void Serializer::ObjectSerializer::VisitInternalReference(RelocInfo* rinfo) {
   // We can only reference to internal references of code that has been output.
   DCHECK(object_->IsCode() && code_has_been_output_);
   // We do not use skip from last patched pc to find the pc to patch, since
   // target_address_address may not return addresses in ascending order when
   // used for internal references. External references may be stored at the
@@ skipping 159 matching lines @@
   if (to_skip != 0 && return_skip == kIgnoringReturn) {
     sink_->Put(kSkip, "Skip");
     sink_->PutInt(to_skip, "SkipDistance");
     to_skip = 0;
   }
   return to_skip;
 }
 
 }  // namespace internal
 }  // namespace v8