Ensure we don't AddLocalReference a bogus value when returning to native code from an upcall that threw.

Also add a few missing CHECKs.

Change-Id: Icf29506b258a2177b5e80c75cd2710761431ba4b
diff --git a/src/oat/runtime/x86/context_x86.h b/src/oat/runtime/x86/context_x86.h
index 72dc719..845f6c3 100644
--- a/src/oat/runtime/x86/context_x86.h
+++ b/src/oat/runtime/x86/context_x86.h
@@ -45,6 +45,7 @@
     return gprs_[reg];
   }
 
+  virtual void SmashCallerSaves();
   virtual void DoLongJump();
 
  private: