Fix free-after-use bug in server callback handler (#27187)

Successful asan run https://source.cloud.google.com/results/invocations/33bc587c-a440-45bd-9886-a23789645a7d/targets

Successful TGP available upon request.

include/grpcpp/impl/codegen/server_callback_handlers.h

@@ -81,7 +81,7 @@ class CallbackUnaryHandler : public ::grpc::internal::MethodHandler {
     ::grpc::ByteBuffer buf;
     buf.set_buffer(req);
     RequestType* request = nullptr;
-    MessageHolder<RequestType, ResponseType>* allocator_state = nullptr;
+    MessageHolder<RequestType, ResponseType>* allocator_state;
     if (allocator_ != nullptr) {
       allocator_state = allocator_->AllocateMessages();
     } else {
@@ -98,8 +98,6 @@ class CallbackUnaryHandler : public ::grpc::internal::MethodHandler {
     if (status->ok()) {
       return request;
     }
-    // Clean up on deserialization failure.
-    allocator_state->Release();
     return nullptr;
   }