avutil/imgutils: Simplify pix_fmt validity check in av_image_get_linesize()

Found-by: Daemon404
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/imgutils.c

@@ -78,7 +78,7 @@ int av_image_get_linesize(enum AVPixelFormat pix_fmt, int width, int plane)
     int max_step     [4];       /* max pixel step for each plane */
     int max_step_comp[4];       /* the component for each plane which has the max pixel step */
 
-    if ((unsigned)pix_fmt >= AV_PIX_FMT_NB || desc->flags & AV_PIX_FMT_FLAG_HWACCEL)
+    if (!desc || desc->flags & AV_PIX_FMT_FLAG_HWACCEL)
         return AVERROR(EINVAL);
 
     av_image_fill_max_pixsteps(max_step, max_step_comp, desc);