I (and dionoea too, I think) encountered an "assertion failed self ==
0" when trying to play a file on win32. I think the following patch
solves this issue. If I understand correcly, the intended behaviour
was to zero the owner field of the mutex when releasing it.

Modified by Courmisch to use InterlockedExchange() directly.
Signed-off-by: Rémi Denis-Courmont <rdenis@simphalempin.com>