Fix crashes.

The bug was due to a refactoring error last night. We will also take out
_artInvokeCommon and other confusing de-dup in the next CL. But I want
to fix the crash right away.

Change-Id: Ic3fcb0bbaaca44cf368f79994a0cdcfac5fdfc8b
diff --git a/src/runtime_support_common.h b/src/runtime_support_common.h
index d4da2de..adab936 100644
--- a/src/runtime_support_common.h
+++ b/src/runtime_support_common.h
@@ -282,6 +282,9 @@
                          Thread* self, Method** sp, bool access_check, InvokeType type){
   Method* method = _artInvokeCommon(method_idx, this_object, caller_method,
                                     self, sp, access_check, type);
+  if (method == NULL) {
+    return 0;  // failure
+  }
   const void* code = method->GetCode();
 
   uint32_t method_uint = reinterpret_cast<uint32_t>(method);