Do not try to read 16bit gray png files with alpha channel.

FFmpeg does not support gray16a.
Fixes the crash in ticket #644.
(cherry picked from commit 0c5fd6372e)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pngdec.c

@@ -471,7 +471,8 @@ static int decode_frame(AVCodecContext *avctx,
                     avctx->pix_fmt = PIX_FMT_MONOBLACK;
                 } else if (s->color_type == PNG_COLOR_TYPE_PALETTE) {
                     avctx->pix_fmt = PIX_FMT_PAL8;
-                } else if (s->color_type == PNG_COLOR_TYPE_GRAY_ALPHA) {
+                } else if (s->bit_depth == 8 &&
+                           s->color_type == PNG_COLOR_TYPE_GRAY_ALPHA) {
                     avctx->pix_fmt = PIX_FMT_GRAY8A;
                 } else {
                     goto fail;