[gh-177] Fix a crash at shutdown on an error in #! script.

When panic() is called from a fiber != sched, we
call atexit handlers and free all memory, including
runtime memory, including the memory used for fiber
stack.

Thus essentially freeing memory the fiber's stack is
using.

The very next instruction using the stack gets
a SIGSEGV afterwards.

Perhaps don't be so strict about freeing resources at
shutdown and do not shoot yourself in the foot.