tests/audiomatch: Fix compile warning

Making sure bestpos is initialized to zero to prevent gcc from kvetching.
It's harmless (although it's not obvious that it's harmless)
from code inspection:
tests/audiomatch.c: In function ‘main’:
tests/audiomatch.c:40: warning: ‘bestpos’ may be used uninitialized in this function

Thanks to Moritz Barsnick for first bringing this to the attention.

Signed-off-by: LiuQi <liuqi@gosun.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
