avcodec/wmalosslessdec: move channel check up

Fixes: out of array access
Fixes: 2nd part of 18429/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMALOSSLESS_fuzzer-6210814364614656

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 891bcc4acc)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wmalosslessdec.c

@@ -190,6 +190,16 @@ static av_cold int decode_init(AVCodecContext *avctx)
         return AVERROR(EINVAL);
     }
 
+    if (avctx->channels < 0) {
+        av_log(avctx, AV_LOG_ERROR, "invalid number of channels %d\n",
+               avctx->channels);
+        return AVERROR_INVALIDDATA;
+    } else if (avctx->channels > WMALL_MAX_CHANNELS) {
+        avpriv_request_sample(avctx,
+                              "More than %d channels", WMALL_MAX_CHANNELS);
+        return AVERROR_PATCHWELCOME;
+    }
+
     s->avctx = avctx;
     ff_llauddsp_init(&s->dsp);
     init_put_bits(&s->pb, s->frame_data, MAX_FRAMESIZE);
@@ -263,16 +273,6 @@ static av_cold int decode_init(AVCodecContext *avctx)
                 ++s->lfe_channel;
     }
 
-    if (s->num_channels < 0) {
-        av_log(avctx, AV_LOG_ERROR, "invalid number of channels %"PRId8"\n",
-               s->num_channels);
-        return AVERROR_INVALIDDATA;
-    } else if (s->num_channels > WMALL_MAX_CHANNELS) {
-        avpriv_request_sample(avctx,
-                              "More than %d channels", WMALL_MAX_CHANNELS);
-        return AVERROR_PATCHWELCOME;
-    }
-
     s->frame = av_frame_alloc();
     if (!s->frame)
         return AVERROR(ENOMEM);