smacker: Check that the data size is a multiple of a sample vector

Fixes out of array access
Fixes: ce19e41f0ef1e52a23edc488faecdb58/asan_heap-oob_2504e97_4202_ffa0df1baed14022b9bfd4f8ac23d0cb.smk

Bug-Id: CVE-2015-8365
CC: libav-stable@libav.org

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 4a9af07a49)
Signed-off-by: Diego Biurrun <diego@biurrun.de>

libavcodec/smacker.c

@@ -636,6 +636,11 @@ static int smka_decode_frame(AVCodecContext *avctx, void *data,
         av_log(avctx, AV_LOG_ERROR, "sample format mismatch\n");
         return AVERROR(EINVAL);
     }
+    if (unp_size % (avctx->channels * (bits + 1))) {
+        av_log(avctx, AV_LOG_ERROR,
+               "The buffer does not contain an integer number of samples\n");
+        return AVERROR(EINVAL);
+    }
 
     /* get output buffer */
     frame->nb_samples = unp_size / (avctx->channels * (bits + 1));