Fix occasional crash when lithium goes out of vregs.

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
diff --git a/src/arm/lithium-arm.cc b/src/arm/lithium-arm.cc
index 772a2cf021a52fafae1b51930aedd267e916611b..4768cc82a41fa68a0cee12d94f0624f4a8c84ccb 100644
--- a/src/arm/lithium-arm.cc
+++ b/src/arm/lithium-arm.cc
@@ -677,7 +677,7 @@ 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;