Fix UnsetM with global base

If the global was not defined, it would try to return
a pointer into the MInstrCtx, which was not passed in.

We dont actually need it though, because in that case we can
just return a pointer to init_null_variant.

hphp/runtime/vm/translator/hopt/vectortranslator.cpp

@@ -600,8 +600,7 @@ static inline TypedValue* baseGImpl(TypedValue *key,
       varEnv->set(name, &tv);
       base = varEnv->lookup(name);
     } else {
-      tvWriteNull(&mis->tvScratch);
-      base = &mis->tvScratch;
+      return const_cast<TypedValue*>(init_null_variant.asTypedValue());
     }
   }
   decRefStr(name);

hphp/test/vm/unset_undef_g.php

@@ -0,0 +1,5 @@
+<?php
+function test($t) {
+  unset($GLOBALS['foo']['bar']);
+}
+test(null);