gpgscm: Build well even if NDEBUG defined.

* gpgscm/scheme.c (gc_reservation_failure): Fix adding ";".
[!NDEBUG] (scheme_init_custom_alloc): Don't init seserved_lineno.

In some build environment, NDEBUG is defined (although it's
bad practice). This change supports such a situation.

• GnuPG-bug-id: T3959
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>