avutil/pixdesc: dont try to use av_read_image_line() with bayer formats

It has undefined behavior ATM as its not supported.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/pixdesc.c

@@ -1986,9 +1986,9 @@ void ff_check_pixfmt_descriptors(void){
             } else {
                 av_assert0(8*(c->step_minus1+1) >= c->depth_minus1+1);
             }
-            av_read_image_line(tmp, (void*)data, linesize, d, 0, 0, j, 2, 0);
             if (!strncmp(d->name, "bayer_", 6))
                 continue;
+            av_read_image_line(tmp, (void*)data, linesize, d, 0, 0, j, 2, 0);
             av_assert0(tmp[0] == 0 && tmp[1] == 0);
             tmp[0] = tmp[1] = (1<<(c->depth_minus1 + 1)) - 1;
             av_write_image_line(tmp, data, linesize, d, 0, 0, j, 2);