loader/dri3: remove a no-op free call

When modifiers == NULL, the free(modifiers) is a no-op, so we can just
remove it

Signed-off-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17974>

src/loader/loader_dri3_helper.c

@@ -1476,7 +1476,6 @@ dri3_alloc_render_buffer(struct loader_dri3_drawable *draw, unsigned int format,
             count = mod_reply->num_screen_modifiers;
             modifiers = malloc(count * sizeof(uint64_t));
             if (!modifiers) {
-               free(modifiers);
                free(mod_reply);
                goto no_image;
             }