muntrace: reset file and hooks before finalizing the stream

muntrace: reset file and hooks before finalizing the stream

fclose will call free, invoking its hook, then fprintf which would indirectly
try to allocate a buffer, and this can cause malloc to be used (thus its hook
to be invoked) if libio uses malloc instead of mmap; given any malloc/free hook
locks the internal lock, this leads to a deadlock.

To prevent this hook roundtrip at muntrace, first unset MALLSTREAM and the
hooks, and only after that close the trace file.