Pass GLIB_DOMAIN as nul-terminated string

[swsnr]

Fixes #1538

[swsnr]

Tested with the following example:

const G_LOG_DOMAIN: &str = "LOGTEST";

fn main() {
    static GLIB_LOGGER: glib::GlibLogger = glib::GlibLogger::new(
        glib::GlibLoggerFormat::Structured,
        glib::GlibLoggerDomain::CrateTarget,
    );
    rs_log::set_logger(&GLIB_LOGGER).unwrap();
    rs_log::set_max_level(rs_log::LevelFilter::Debug);

    glib::debug!("DEBUG");
    glib::info!("INFO");
    glib::warn!("WARN");
}

With this PR G_MESSAGES_DEBUG shows all messages:

$ G_MESSAGES_DEBUG=LOGTEST cargo run --features log,log_macros --package glib --example logtest
   Compiling glib v0.21.0 (/home/basti/Code/gtk-rs/gtk-rs-core/glib)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 4.40s
     Running `target/debug/examples/logtest`
(process:66204): LOGTEST-DEBUG: 09:36:13.777: DEBUG
LOGTEST-INFO: 09:36:13.777: INFO

(process:66204): LOGTEST-WARNING **: 09:36:13.777: WARN

Previously, it'd only show the warning message.

If it's any help I can also include the example in the pull request.

[sdroege]

As mentioned in #1538 (comment) this is not the way to go. It would incur an unnecessary performance penalty and shouldn't be necessary