Fix occasional crash when lithium goes out of vregs.

src/mips/lithium-mips.cc

Index: src/mips/lithium-mips.cc
diff --git a/src/mips/lithium-mips.cc b/src/mips/lithium-mips.cc
index 11c620b76c230136ce8f6dc9e6c522c0ad1871bb..4b7294470d84cf08d31624c30c5e478f0f78528a 100644
--- a/src/mips/lithium-mips.cc
+++ b/src/mips/lithium-mips.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;