Allow construction of x64 Assembler with a NULL Isolate.

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index b13fbe852514890c10bd16d59c8e89af297e40ab..4f61b9bd2dbfca9efd686b6e36b32f062962d04c 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -354,7 +354,7 @@ Assembler::Assembler(Isolate* arg_isolate, void* buffer, int buffer_size)
     if (buffer_size <= kMinimalBufferSize) {
       buffer_size = kMinimalBufferSize;
 
-      if (isolate()->assembler_spare_buffer() != NULL) {
+      if (isolate() != NULL && isolate()->assembler_spare_buffer() != NULL) {
         buffer = isolate()->assembler_spare_buffer();
         isolate()->set_assembler_spare_buffer(NULL);
       }
@@ -398,7 +398,8 @@ Assembler::Assembler(Isolate* arg_isolate, void* buffer, int buffer_size)
 
 Assembler::~Assembler() {
   if (own_buffer_) {
-    if (isolate()->assembler_spare_buffer() == NULL &&
+    if (isolate() != NULL && 
+        isolate()->assembler_spare_buffer() == NULL &&
         buffer_size_ == kMinimalBufferSize) {
       isolate()->set_assembler_spare_buffer(buffer_);
     } else {
@@ -521,7 +522,8 @@ void Assembler::GrowBuffer() {
           reloc_info_writer.pos(), desc.reloc_size);
 
   // Switch buffers.
-  if (isolate()->assembler_spare_buffer() == NULL &&
+  if (isolate() != NULL &&
+      isolate()->assembler_spare_buffer() == NULL &&
       buffer_size_ == kMinimalBufferSize) {
     isolate()->set_assembler_spare_buffer(buffer_);
   } else {