avcodec/agm: Fix overflow of signed shift

Fixes: left shift of 1 by 31 places cannot be represented in type 'int'
Fixes: 15328/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AGM_fuzzer-5637545171353600

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

libavcodec/agm.c

@@ -918,7 +918,7 @@ static void get_tree_codes(uint32_t *codes, Node *nodes, int idx, uint32_t pfx,
         codes[idx] = pfx;
     } else if (idx >= 0) {
         get_tree_codes(codes, nodes, nodes[idx].child[0], pfx + (0 << bitpos), bitpos + 1);
-        get_tree_codes(codes, nodes, nodes[idx].child[1], pfx + (1 << bitpos), bitpos + 1);
+        get_tree_codes(codes, nodes, nodes[idx].child[1], pfx + (1U << bitpos), bitpos + 1);
     }
 }