egl/surfaceless: Fix segfault in eglSwapBuffers

Since commit 63c5d5c6c46c8472ee7a8241a0f80f13d79cb8cd, the surfaceless
platform has allowed creation of pbuffer surfaces. But the vtable entry
for eglSwapBuffers has remained NULL.

Discovered by running a little pbuffer test.

Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
1 file changed