gifenc: support resolutions up to 65535x65535

Maybe someone can add a check in the second gif encoder (rgb24), because I'm not sure where it should be added.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/gif.c

@@ -145,6 +145,11 @@ static av_cold int gif_encode_init(AVCodecContext *avctx)
 {
     GIFContext *s = avctx->priv_data;
 
+    if (avctx->width > 65535 || avctx->height > 65535) {
+        av_log(avctx, AV_LOG_ERROR, "GIF does not support resolutions above 65535x65535\n");
+        return -1;
+    }
+
     avctx->coded_frame = &s->picture;
     s->lzw = av_mallocz(ff_lzw_encode_state_size);
     if (!s->lzw)