on2avc: check number of channels

Fixes invalid memory access.

CC: libav-stable@libav.org
Bug-ID: CVE-2014-8549
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit cee4490b52)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/on2avc.c

@@ -918,6 +918,10 @@ static av_cold int on2avc_decode_init(AVCodecContext *avctx)
         av_log(avctx, AV_LOG_ERROR, "0x500 version should be mono\n");
         return AVERROR_INVALIDDATA;
     }
+    if (avctx->channels > 2) {
+        av_log(avctx, AV_LOG_ERROR, "Only 1 or 2 channels are supported.\n");
+        return AVERROR(EINVAL);
+    }
     if (avctx->channels == 2)
         av_log(avctx, AV_LOG_WARNING,
                "Stereo mode support is not good, patch is welcome\n");