After causing a deadlock in an after-fork handler installed with pthread_atfork, an attempt to unwind that process with pystack remote --native-all is giving me

Engine error: basic_string::_S_construct null not valid

That's happening because dwfl_getthread_frames is not finding any frames, and also not setting dwfl_errno to something non-zero. Interestingly, this doesn't seem to reproduce with eu-stack, so we might be doing something wrong here that's causing this.

#101 fixes the failure mode that we get here, but we should figure out why unwinding is failing, as both gdb and eu-stack succeed.