avcodec/pcm: Fix undefined shifts

Fixes the acodec-pcm-u16[lb]e FATE-tests.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/pcm.c

@@ -303,7 +303,7 @@ static av_cold int pcm_decode_close(AVCodecContext *avctx)
 #define DECODE(size, endian, src, dst, n, shift, offset)                \
     for (; n > 0; n--) {                                                \
         uint ## size ## _t v = bytestream_get_ ## endian(&src);         \
-        AV_WN ## size ## A(dst, (v - offset) << shift);                 \
+        AV_WN ## size ## A(dst, (uint ## size ## _t)(v - offset) << shift); \
         dst += size / 8;                                                \
     }
 
@@ -314,7 +314,7 @@ static av_cold int pcm_decode_close(AVCodecContext *avctx)
         dst = frame->extended_data[c];                                \
         for (i = n; i > 0; i--) {                                       \
             uint ## size ## _t v = bytestream_get_ ## endian(&src);     \
-            AV_WN ## size ## A(dst, (v - offset) << shift);             \
+            AV_WN ## size ## A(dst, (uint ## size ##_t)(v - offset) << shift); \
             dst += size / 8;                                            \
         }                                                               \
     }