hevc: do not dereference pointer before NULL check in verify_md5()

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/hevc.c

@@ -2765,12 +2765,14 @@ static void print_md5(void *log_ctx, int level, uint8_t md5[16])
 static int verify_md5(HEVCContext *s, AVFrame *frame)
 {
     const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(frame->format);
-    int pixel_shift = desc->comp[0].depth_minus1 > 7;
+    int pixel_shift;
     int i, j;
 
     if (!desc)
         return AVERROR(EINVAL);
 
+    pixel_shift = desc->comp[0].depth_minus1 > 7;
+
     av_log(s->avctx, AV_LOG_DEBUG, "Verifying checksum for frame with POC %d: ",
            s->poc);