Prevent segfault when colord fails with no reported error

This can occur due to assertion errors for example.

Program received signal SIGSEGV, Segmentation fault.
0x00007fffd4de5a92 in gcm_session_client_connect_cb (source_object=0x6c3920, res=0x63e0b0,
    user_data=0x7fffe4005ce0) at gsd-color-profiles.c:85
85	                g_warning ("failed to add user icc: %s", error->message);