avcodec/amr_parser: Treat 0 channels as 1

Fixes: division by 0
Fixes: 39562/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AMRWB_fuzzer-5448834960982016
Fixes: 39589/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AMRWB_fuzzer-6119205334810624

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

libavcodec/amr_parser.c

@@ -25,6 +25,7 @@
  * Splits packets into individual blocks.
  */
 
+#include "libavutil/channel_layout.h"
 #include "libavutil/intreadwrite.h"
 #include "parser.h"
 
@@ -62,6 +63,11 @@ static int amr_parse(AVCodecParserContext *s1,
     *poutbuf_size = 0;
     *poutbuf = NULL;
 
+    if (!avctx->channels) {
+        avctx->channels       = 1;
+        avctx->channel_layout = AV_CH_LAYOUT_MONO;
+    }
+
     if (s1->flags & PARSER_FLAG_COMPLETE_FRAMES) {
         next = buf_size;
     } else {