wav: init st to NULL to avoid a false-positive warning.

If st is NULL, it means no 'fmt ' tag is found, but 'data' tag (which
needs a previous 'fmt ' tag to be parsed correctly and st initialized)
check will make sure st is never dereferenced in that case.

Fixes warning:
    libavformat/wav.c: In function ‘wav_read_header’:
    libavformat/wav.c:499:44: warning: ‘st’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
