avcodec/adpcm: Fix integer overflow in ADPCM THP

The reference (thp.txt) uses floats so wrap around would seem incorrect.

Fixes: signed integer overflow: 1073741824 + 1073741824 cannot be represented in type 'int'
Fixes: 20658/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ADPCM_THP_fuzzer-5646302555930624

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

libavcodec/adpcm.c

@@ -1853,8 +1853,8 @@ static int adpcm_decode_frame(AVCodecContext *avctx, void *data,
                 int byte = bytestream2_get_byteu(&gb);
                 int index = (byte >> 4) & 7;
                 unsigned int exp = byte & 0x0F;
-                int factor1 = table[ch][index * 2];
-                int factor2 = table[ch][index * 2 + 1];
+                int64_t factor1 = table[ch][index * 2];
+                int64_t factor2 = table[ch][index * 2 + 1];
 
                 /* Decode 14 samples.  */
                 for (n = 0; n < 14 && (i * 14 + n < nb_samples); n++) {