cleanup the mutex use of PerlIOStdio_close

cleanup the mutex use of PerlIOStdio_close

Before: clang -Wthread-safety found the flow quite suspect:
one conditional mutex lock, with two conditional unlocks.
The code *looked* like the being okay logic-wise now, but
rather fragile, so let's make it clearer and more solid
by hoisting the locking earlier.

This is a fd close, this should not be a performance sensitive spot.

And it's in PerlIOStdio, so it should be pretty dead anyway.

perlio.c:3283:18: warning: mutex 'PL_perlio_mutex' is not held on every path through here [-Wthread-safety-analysis]
        result = PerlSIO_fclose(stdio);

perlio.c:3299:6: warning: releasing mutex 'PL_perlio_mutex' that was not held [-Wthread-safety-analysis]
            MUTEX_UNLOCK(&PL_perlio_mutex);