avcodec/loco: Fix integer overflow with large values from loco_get_rice()

Fixes: signed integer overflow: 155 + 2147483647 cannot be represented in type 'int'
Fixes: 23421/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_LOCO_fuzzer-5652849097965568

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

libavcodec/loco.c

@@ -131,7 +131,7 @@ static int loco_decode_plane(LOCOContext *l, uint8_t *data, int width, int heigh
                              int stride, const uint8_t *buf, int buf_size)
 {
     RICEContext rc;
-    int val;
+    unsigned val;
     int ret;
     int i, j;