avfilter/af_pan: check nb_output_channels before use

Fixes: CID1500281 Out-of-bounds write
Fixes: CID1500331 Out-of-bounds write

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 5fe8bf4aa5)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/af_pan.c

@@ -118,6 +118,14 @@ static av_cold int init(AVFilterContext *ctx)
     if (ret < 0)
         goto fail;
 
+    if (pan->nb_output_channels > MAX_CHANNELS) {
+        av_log(ctx, AV_LOG_ERROR,
+               "af_pan supports a maximum of %d channels. "
+               "Feel free to ask for a higher limit.\n", MAX_CHANNELS);
+        ret = AVERROR_PATCHWELCOME;
+        goto fail;
+    }
+
     /* parse channel specifications */
     while ((arg = arg0 = av_strtok(NULL, "|", &tokenizer))) {
         int used_in_ch[MAX_CHANNELS] = {0};