egl_dri2: NULL check for xcb_dri2_get_buffers_reply()
egl_dri2: NULL check for xcb_dri2_get_buffers_reply()

Without the check, unsuccessful xcb_dri2_get_buffers_reply(...) causes
segmentation fault in dri2_get_buffers.

Signed-off-by: Matt Roper <f1605a76cb9f6387d99228a0a9260318379b62de@intel.com>
Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler