UBSan: invalid vptr in when custom_rotating_file_sink::_sink_it fails

/third_party/spdlog/include/spdlog/common.h:139:9: runtime error: member access within address 0x000101502268 which does not point to an object of type 'spdlog::spdlog_ex'

0x000101502268: note: object has invalid vptr

<memory cannot be printed>

* thread #2, stop reason = signal SIGABRT

  * frame #0: 0x00007fff55e62e3e libsystem_kernel.dylib` __pthread_kill  + 10

    frame #1: 0x00007fff55fa1150 libsystem_pthread.dylib` pthread_kill  + 333

    frame #2: 0x00007fff55dbf312 libsystem_c.dylib` abort  + 127

    frame #3: 0x0000000100809916 libclang_rt.ubsan_osx_dynamic.dylib` __sanitizer::Abort()  + 70

    frame #4: 0x0000000100804c08 libclang_rt.ubsan_osx_dynamic.dylib` __sanitizer::Die()  + 120

    frame #5: 0x00000001007eb48d libclang_rt.ubsan_osx_dynamic.dylib` __ubsan::ScopedReport::~ScopedReport()  + 461

    frame #6: 0x00000001007f0aeb libclang_rt.ubsan_osx_dynamic.dylib` HandleDynamicTypeCacheMiss(__ubsan::DynamicTypeCacheMissData*, unsigned long, unsigned long, __ubsan::ReportOptions)  + 1947

    frame #7: 0x00000001007f0344 libclang_rt.ubsan_osx_dynamic.dylib` __ubsan_handle_dynamic_type_cache_miss  + 68

    frame #8: 0x00000001004f55ca libmemcached_logger.1.0.0.dylib` spdlog::spdlog_ex::spdlog_ex(this=0x0000000101502268, msg=<unavailable>, last_errno=60)  + 618 at common.h:139

    frame #9: 0x00000001004f1617 libmemcached_logger.1.0.0.dylib` spdlog::details::file_helper::open(this=<unavailable>, fname=<unavailable>, truncate=<unavailable>)  + 663 at file_helper.h:60

    frame #10: 0x00000001004f2911 libmemcached_logger.1.0.0.dylib` custom_rotating_file_sink<std::__1::mutex>::_sink_it(this=0x000000010223c0f8, msg=<unavailable>)  + 481 at custom_rotating_file_sink.cc:85