upb:file_descriptor_parsenew_fuzzer: Bad-cast to upb_OneofDef' (aka 'struct upb_OneofDef')create_fielddef in create_msgdef

Detailed Report: https://oss-fuzz.com/testcase?key=4733587960758272 Project: upb Fuzzing Engine: libFuzzer Fuzz Target: filedescriptorparsenewfuzzer Job Type: libfuzzerubsanupb Platform Id: linux Crash Type: Bad-cast Crash Address: 0xffffffe126cd9680 Crash State: Bad-cast to upbOneofDef' aka...