Throw away uncleanable scopes when exiting a pseudo-forked process.

Throw away uncleanable scopes when exiting a pseudo-forked process.

Commit adab9969 tried to clean up those additional scopes, but failed
because some of the memory was allocated from a different pool. To
avoid triggering the assert() in perl_destruct() this change instead
moves the one remaining scope back to the root of the stack, effectively
discarding the additional frames without any further processing.