Fixed crashes exposed though fuzzing.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 61211ec6cbc53e3562cbb862c00432ac6d5e7667..2d8f5f0c6b2aceb8e86eee1dff0be4557739e0a4 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -9715,9 +9715,10 @@ void HOptimizedGraphBuilder::GenerateOneByteSeqStringSetChar(
   HValue* value = Pop();
   HValue* index = Pop();
   HValue* string = Pop();
-  HSeqStringSetChar* result = New<HSeqStringSetChar>(
-      String::ONE_BYTE_ENCODING, string, index, value);
-  return ast_context()->ReturnInstruction(result, call->id());
+  Add<HSeqStringSetChar>(String::ONE_BYTE_ENCODING, string,
+                         index, value);
+  Add<HSimulate>(call->id(), FIXED_SIMULATE);
+  return ast_context()->ReturnValue(graph()->GetConstantUndefined());
 }
 
 
@@ -9730,9 +9731,10 @@ void HOptimizedGraphBuilder::GenerateTwoByteSeqStringSetChar(
   HValue* value = Pop();
   HValue* index = Pop();
   HValue* string = Pop();
-  HSeqStringSetChar* result = New<HSeqStringSetChar>(
-      String::TWO_BYTE_ENCODING, string, index, value);
-  return ast_context()->ReturnInstruction(result, call->id());
+  Add<HSeqStringSetChar>(String::TWO_BYTE_ENCODING, string,
+                         index, value);
+  Add<HSimulate>(call->id(), FIXED_SIMULATE);
+  return ast_context()->ReturnValue(graph()->GetConstantUndefined());
 }