loader: fix close on uninitialized file descriptor value

Using a drm syscall layer faking a kernel driver :

  ==581460== Conditional jump or move depends on uninitialised value(s)
  ==581460==    by 0x48A4C2B: close (drm-hooks.cpp:185)
  ==581460==    by 0x5A815F1: dri3_alloc_render_buffer (loader_dri3_helper.c:1469)
  ==581460==    by 0x5A82050: dri3_get_buffer (loader_dri3_helper.c:1827)
  ==581460==    by 0x5A82662: loader_dri3_get_buffers (loader_dri3_helper.c:2028)
  ==581460==    by 0x6C78109: intel_update_image_buffers (brw_context.c:1870)
  ==581460==    by 0x6C77805: intel_update_renderbuffers (brw_context.c:1499)
  ==581460==    by 0x6C7789D: intel_prepare_render (brw_context.c:1520)
  ==581460==    by 0x6C773D4: intelMakeCurrent (brw_context.c:1341)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 069fdd5f ("egl/x11: Support DRI3 v1.1")
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3152>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3152>
(cherry picked from commit fc2552b6)