egl/glvnd: correctly report errors when vendor cannot be found

egl/glvnd: correctly report errors when vendor cannot be found

If the user provides an invalid display or device the ToVendor lookup
will fail.

In this case, the local [Mesa vendor] error code will be set. Thus on
sequential eglGetError(), the error will be EGL_SUCCESS.

To be more specific, GLVND remembers the last vendor and calls back
into it's eglGetError, although there's no guarantee to ever have had
one.

v2:
 - Add _eglError call, so the debug callback is executed (Kyle)
 - Drop XXX comment.

Piglit: tests/egl/spec/egl_ext_device_query
Fixes: ce562f9e3fa ("EGL: Implement the libglvnd interface for EGL (v3)")
Cc: Eric Engestrom <eric@engestrom.ch>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Kyle Brenneman <kbrenneman@nvidia.com>