avcodec/pcm: Fix invalid shift in pcm_decode_frame for LXF

Fixes: left shift of 32 by 28 places cannot be represented in type 'int'
Fixes: 19472/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PCM_LXF_fuzzer-5704364320096256

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pcm.c

@@ -515,7 +515,7 @@ static int pcm_decode_frame(AVCodecContext *avctx, void *data,
             dst_int32_t = (int32_t *)frame->extended_data[c];
             for (i = 0; i < n; i++) {
                 // extract low 20 bits and expand to 32 bits
-                *dst_int32_t++ =  (src[2]         << 28) |
+                *dst_int32_t++ =  ((uint32_t)src[2]<<28) |
                                   (src[1]         << 20) |
                                   (src[0]         << 12) |
                                  ((src[2] & 0x0F) <<  8) |