Move collection reset before unref (since unref could destroy obj)

include/grpc++/impl/codegen/call.h

@@ -610,10 +610,12 @@ class CallOpSet : public CallOpSetInterface,
     this->Op5::FinishOp(status);
     this->Op6::FinishOp(status);
     *tag = return_tag_;
-    g_core_codegen_interface->grpc_call_unref(call_);
+
     // TODO(vjpai): Remove the reference to collection_ once the idea of
     // bypassing the code generator is forbidden. It is already deprecated
     collection_.reset();
+
+    g_core_codegen_interface->grpc_call_unref(call_);
     return true;
   }