The error cases (goto fin) of _nsdispatch were missing the unlock.

This change also drops the checks for __isthreaded since the pthread stubs
are already no-ops if threads are not being used. Dropping those conditionals
allows clang's thread safety analysis to deal with the file and also makes
the code a bit more readable. While touching the file also add a few more
assertions in debug mode that the right locks are held.