apedec: fix signed integer overflows

This bit manipulation is equivalent but avoids undefined
shifts and overflows.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/apedec.c

@@ -690,7 +690,7 @@ 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;