Crash from calling XDefaultScreen on null pointer

[riking]

https://asan.saethlin.dev/ub?crate=user-idle&version=0.5.3

running 1 test
test src/lib.rs - (line 8) ... FAILED

failures:

---- src/lib.rs - (line 8) stdout ----
Test executable failed (exit status: 1).

stderr:
AddressSanitizer:DEADLYSIGNAL
=================================================================
==4086==ERROR: AddressSanitizer: SEGV on unknown address 0x0000000000e0 (pc 0x7fe7ddb22b14 bp 0x7fff21f88530 sp 0x7fff21f88458 T0)
==4086==The signal is caused by a READ memory access.
==4086==Hint: address points to the zero page.
    #0 0x7fe7ddb22b14 in XDefaultScreen (/lib/x86_64-linux-gnu/libX11.so.6+0x29b14) (BuildId: d1d3345a252a40a004cbd02011a651930e172ccd)
    #1 0x556b394e4bb4 in user_idle::x11_impl::get_idle_time::hf512c1deaede5145 /build/src/x11_impl.rs:16:22
    #2 0x556b394e4f21 in user_idle::UserIdle::get_time::hea02ab867f8d08bb /build/src/lib.rs:42:24
    #3 0x556b394e35c8 in rust_out::main::_doctest_main_src_lib_rs_8_0::hcb90bc0325d9a8c6 /build/src/lib.rs:6:12
    #4 0x556b394e34d5 in rust_out::main::h3c365c6a42822ba3 /build/src/lib.rs:10:3

XDefaultScreen is called on a null pointer.