avcodec/qdm2: error out of qdm2_fft_decode_tones() before entering endless loop

Fixes: signed integer overflow: 2147483646 + 2 cannot be represented in type 'int'
Fixes: infinite loop
Fixes: 15396/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_QDM2_fuzzer-5116605501014016

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 694be24bd6)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/qdm2.c

@@ -1289,6 +1289,10 @@ static void qdm2_fft_decode_tones(QDM2Context *q, int duration,
             }
             offset += (n - 2);
         } else {
+            if (local_int_10 <= 2) {
+                av_log(NULL, AV_LOG_ERROR, "qdm2_fft_decode_tones() stuck\n");
+                return;
+            }
             offset += qdm2_get_vlc(gb, &vlc_tab_fft_tone_offset[local_int_8], 1, 2);
             while (offset >= (local_int_10 - 1)) {
                 offset       += (1 - (local_int_10 - 1));