ucm: parser: Fix snprintf usage

ucm: parser: Fix snprintf usage

There is no need to manually 0 terminate the buffer with snprintf,
only strncpy has the very unfortunate behavior of not guaranteeing 0
termination.

Likewise there is no need to substract one from the buffer size of
the buffer passed to snprintf.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>