Fix: translate-trace-ir-to-ctf-ir.c: dereference after `NULL` check

Fix: translate-trace-ir-to-ctf-ir.c: dereference after `NULL` check

We only need to call must_protect_variant_option_name() when the tag
field class is not created by the component (`fc->tag_is_before`).

Furthermore, the local `must_protect` variable is not used, so the
corresponding must_protect_variant_option_name() parameter is not
either. They are both removed. Because the
must_protect_variant_option_name() function actually sets the name of
the option to use (protecting it or not), it's renamed to
maybe_protect_variant_option_name().

Found by Coverity Scan.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ieccc0bcddc5570f473d4303f7cc470a56af65a9e