avcodec/apedec: fix undefined left shifts of negative numbers

This fixes -Wshift-negative-value reported with clang 3.7+, e.g
http://fate.ffmpeg.org/log.cgi?time=20150919172459&log=compile&slot=x86_64-darwin-clang-polly-notiling-3.7.
Note that the patch crucially depends on int >= 32 bits,
an assumption made in many places in the codebase.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/apedec.c

@@ -1281,7 +1281,7 @@ static void do_apply_filter(APEContext *ctx, int version, APEFilter *f,
             /* Update the adaption coefficients */
             absres = FFABS(res);
             if (absres)
-                *f->adaptcoeffs = ((res & (-1<<31)) ^ (-1<<30)) >>
+                *f->adaptcoeffs = ((res & (-(1<<31))) ^ (-(1<<30))) >>
                                   (25 + (absres <= f->avg*3) + (absres <= f->avg*4/3));
             else
                 *f->adaptcoeffs = 0;