avcodec/g726: init missing sample rate

Fixes: 416134551/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ADPCM_G726_DEC_fuzzer-5695764455292928
Found-by: OSS-Fuzz
Signed-off-by: Kacper Michajłow <kasper93@gmail.com>
diff --git a/libavcodec/g726.c b/libavcodec/g726.c
index 5491b7e..f41df30 100644
--- a/libavcodec/g726.c
+++ b/libavcodec/g726.c
@@ -455,6 +455,8 @@
     g726_reset(c);
 
     avctx->sample_fmt = AV_SAMPLE_FMT_S16;
+    if (!avctx->sample_rate)
+        avctx->sample_rate = 8000;
 
     return 0;
 }