pipe-loader: check that the pointer to driconf_xml isn't NULL

This happens when mesa is built with only swrast. The default
driver being kmsro and the default driconf file being v3d,
it's NULL and then strdup crashes.

This fixes a crash with piglit spec/egl_mesa_query_driver/conformance.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>

src/gallium/auxiliary/pipe-loader/pipe_loader_drm.c

@@ -331,7 +331,7 @@ pipe_loader_drm_get_driinfo_xml(const char *driver_name)
    const struct drm_driver_descriptor *dd =
       get_driver_descriptor(driver_name, &lib);
 
-   if (dd && dd->driconf_xml)
+   if (dd && dd->driconf_xml && *dd->driconf_xml)
       xml = strdup(*dd->driconf_xml);
 
    if (lib)