Fixed crashes exposed though fuzzing.

src/x64/macro-assembler-x64.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 774 matching lines @@
       Label::Distance near_jump = Label::kFar);
 
   void JumpIfBothInstanceTypesAreNotSequentialAscii(
       Register first_object_instance_type,
       Register second_object_instance_type,
       Register scratch1,
       Register scratch2,
       Label* on_fail,
       Label::Distance near_jump = Label::kFar);
 
+  void EmitSeqStringSetCharCheck(Register string,
+                                 Register index,
+                                 Register value,
+                                 uint32_t encoding_mask);
+
   // Checks if the given register or operand is a unique name
   void JumpIfNotUniqueName(Register reg, Label* not_unique_name,
                            Label::Distance distance = Label::kFar);
   void JumpIfNotUniqueName(Operand operand, Label* not_unique_name,
                            Label::Distance distance = Label::kFar);
 
   // ---------------------------------------------------------------------------
   // Macro instructions.
 
   // Load/store with specific representation.
@@ skipping 246 matching lines @@
   // Unlink the stack handler on top of the stack from the try handler chain.
   void PopTryHandler();
 
   // Activate the top handler in the try hander chain and pass the
   // thrown value.
   void Throw(Register value);
 
   // Propagate an uncatchable exception out of the current JS stack.
   void ThrowUncatchable(Register value);
 
+  // Throw a message string as an exception.
+  void Throw(BailoutReason reason);
+
+  // Throw a message string as an exception if a condition is not true.
+  void ThrowIfNot(Condition cc, BailoutReason reason);
+
   // ---------------------------------------------------------------------------
   // Inline caching support
 
   // Generate code for checking access rights - used for security checks
   // on access to global objects across environments. The holder register
   // is left untouched, but the scratch register and kScratchRegister,
   // which must be different, are clobbered.
   void CheckAccessGlobalProxy(Register holder_reg,
                               Register scratch,
                               Label* miss);
@@ skipping 528 matching lines @@
     masm->popfq();                                                           \
   }                                                                          \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_