Fix occasional crash when lithium goes out of vregs.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index a6f0466b7ad75c0d38982f3ba6dfc4e2ea4ecbda..08e0dae1a15a4fe00bc52b1bfd5631725b5978a6 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -685,7 +685,7 @@ LUnallocated* LChunkBuilder::TempRegister() {
   int vreg = allocator_->GetVirtualRegister();
   if (!allocator_->AllocationOk()) {
     Abort("Out of virtual registers while trying to allocate temp register.");
-    return NULL;
+    vreg = 0;
   }
   operand->set_virtual_register(vreg);
   return operand;