Adjust WIENER_FILT_TAP2_MIDV value to fix convolve

Adjusts the value by 1 to make sure that the center tap
if the Wiener filter does not drop below 0.

BUG=aomedia:315

Change-Id: I41c3a2eb3f36dd49072a4873a995003d18f94ece
1 file changed