mp4: Fix possible memory leak on errors.

mp4: Fix possible memory leak on errors.

If the sanity checks in open_file() fail, we free the mp4 structure
but not the various tables and metadata items we might already have
allocated at this point.

Fix this by calling mp4_close() instead of freeing the mp4 struct
directly. We have to move mp4_close() above open_file() to avoid a
forward declaration.