avcodec/wmalosslessdec: Check channels after setting them

Fixes: 46194/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMALOSSLESS_fuzzer-580292873827123
Fixes: stack-buffer-overflow on address 0x7ffc0ce69b30 at pc 0x00000062fb03 bp 0x7ffc0ce69af0 sp 0x7ffc0ce69ae8
Fixes: 46205/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMALOSSLESS_fuzzer-5354894996930560
Fixes: 47861/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMALOSSLESS_fuzzer-4817404984688640

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

libavcodec/wmalosslessdec.c

@@ -191,13 +191,6 @@ static av_cold int decode_init(AVCodecContext *avctx)
         return AVERROR(EINVAL);
     }
 
-    av_assert0(avctx->ch_layout.nb_channels >= 0);
-    if (avctx->ch_layout.nb_channels > WMALL_MAX_CHANNELS) {
-        avpriv_request_sample(avctx,
-                              "More than " AV_STRINGIFY(WMALL_MAX_CHANNELS) " channels");
-        return AVERROR_PATCHWELCOME;
-    }
-
     if (avctx->extradata_size >= 18) {
         s->decode_flags    = AV_RL16(edata_ptr + 14);
         channel_mask       = AV_RL32(edata_ptr +  2);
@@ -226,6 +219,12 @@ static av_cold int decode_init(AVCodecContext *avctx)
         av_channel_layout_uninit(&avctx->ch_layout);
         av_channel_layout_from_mask(&avctx->ch_layout, channel_mask);
     }
+    av_assert0(avctx->ch_layout.nb_channels >= 0);
+    if (avctx->ch_layout.nb_channels > WMALL_MAX_CHANNELS) {
+        avpriv_request_sample(avctx,
+                            "More than " AV_STRINGIFY(WMALL_MAX_CHANNELS) " channels");
+        return AVERROR_PATCHWELCOME;
+    }
 
     s->num_channels = avctx->ch_layout.nb_channels;