grpc: Heap-use-after-free in post_batch_completion

Detailed report: https://oss-fuzz.com/testcase?key=5965570207907840 Project: grpc Fuzzer: libFuzzergrpcapifuzzer Fuzz target binary: apifuzzer Job Type: libfuzzerasangrpc Platform Id: linux Crash Type: Heap-use-after-free READ 8 Crash Address: 0x6250000c21a0 Crash State: postbatchcompletion...