egl/wayland: bail out when drmGetMagic fails

Currently as the function fails, we pass uninitialized data to the
authentication function. Stop doing that and print an warning when
the function fails.

v2: Plug memory leak in error path (Eric)

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com> (v1)
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit c59d3aa4)