log: fix panic when using a log id with a formatted log message
Would panic because of a missing NUL terminator:
thread '<unnamed>' panicked at .. glib/src/gstring.rs:61:9:
assertion failed: !bytes.is_empty() && bytes[bytes.len() - 1] == 0
3: glib::gstring::GStr::from_utf8_with_nul_unchecked
4: gstreamer::log::DebugCategory::log_id_unfiltered_internal
at gstreamer-rs/gstreamer/src/log.rs:465
5: gstreamer::log::DebugCategory::log_id_unfiltered
at gstreamer-rs/gstreamer/src/log.rs:425