avcodec/mediacodecdec: fix SEGV on modern nvidia decoders

This code came originally from gstreamer, where it was added in [1]
as a work-around for the Tegra 3. (The alignment was changed in [2]
as a response to [3], from 32-bit to 16-bit).

gstreamer only used this workaround in the case where the decoder
didn't return a slice-height property, but when the code was copied
into avcodec the conditional got lost. This commit restores the guard
and prefers the slice-height from the decoder when it is available.

This fixes segfaults decoding 1920x1080 h264 and mpeg2 videos on the
NVidia SHIELD after upgrading to Android Oreo.

[1] https://github.com/GStreamer/gst-plugins-bad/commit/a870e6a5c30dd85240fe75c7409cc1cf1b86541d
[2] https://github.com/GStreamer/gst-plugins-bad/commit/21ff3ae0b0127bd82951d278ca24f2d54133b7cd
[3] https://bugzilla.gnome.org/show_bug.cgi?id=748867Signed-off-by: Aman Gupta <aman@tmm1.net>

libavcodec/mediacodecdec_common.c

@@ -389,13 +389,14 @@ static int mediacodec_dec_parse_format(AVCodecContext *avctx, MediaCodecDecConte
     s->stride = s->stride > 0 ? s->stride : s->width;
 
     AMEDIAFORMAT_GET_INT32(s->slice_height, "slice-height", 0);
-    s->slice_height = s->slice_height > 0 ? s->slice_height : s->height;
 
-    if (strstr(s->codec_name, "OMX.Nvidia.")) {
+    if (strstr(s->codec_name, "OMX.Nvidia.") && s->slice_height == 0) {
         s->slice_height = FFALIGN(s->height, 16);
     } else if (strstr(s->codec_name, "OMX.SEC.avc.dec")) {
         s->slice_height = avctx->height;
         s->stride = avctx->width;
+    } else if (s->slice_height == 0) {
+        s->slice_height = s->height;
     }
 
     AMEDIAFORMAT_GET_INT32(s->color_format, "color-format", 1);