avcodec/mlpdec: fix a undefined left shift of negative number

This fixes a -Wshift-negative-value reported with clang 3.7+, e.g
http://fate.ffmpeg.org/log.cgi?time=20150918181527&log=compile&slot=x86_64-darwin-clang-polly-vectorize-stripmine-3.7.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mlpdec.c

@@ -602,7 +602,7 @@ static int read_restart_header(MLPDecodeContext *m, GetBitContext *gbp,
 
         /* Default audio coding is 24-bit raw PCM. */
         cp->huff_offset      = 0;
-        cp->sign_huff_offset = (-1) << 23;
+        cp->sign_huff_offset = -(1 << 23);
         cp->codebook         = 0;
         cp->huff_lsbs        = 24;
     }