egl: move Null check to eglGetSyncAttribKHR to prevent Segfault

egl: move Null check to eglGetSyncAttribKHR to prevent Segfault

Null-check on "*value" is currently done in _eglGetSyncAttrib, which is
after eglGetSyncAttribKHR dereferences it.

Move the check a layer up (in the beginning of eglGetSyncAttribKHR) to
avoid segfaults.

Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
[Emil Velikov: tweak commit message, add stable tag]
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>