frontends/va: fix coredump as creating surface with VAConfigAttrib

As creating surface with VAConfigAttrib, checking if modifier from attrib list is null

Signed-off-by: shanshengwang <shansheng.wang@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15483>
1 file changed