wmadec: Fix left shift of negative value.

wmadec: Fix left shift of negative value.

gcc-6.1 complains about this:

	wmadec_filter.c:819:33: warning: left shift of negative value [-Wshift-negative-value]
	   mult1 = mult * exponents[((-1 << bsize)) >> esize];

The new code still looks wrong because we now shift a negative value
to the right. Moreover, it is not clear that the resulting value
is within array bounds. On the other hand, ffmpeg has the same fix
(commit a48b24e5 in the ffmpeg repository), so..