audit: tame initialization warning len_abuf in audit_log_execve_info

Tame initialization warning of len_abuf in audit_log_execve_info even
though there isn't presently a bug introduced by commit 43761473
("audit: fix a double fetch in audit_log_single_execve_arg()").  Using
UNINITIALIZED_VAR instead may mask future bugs.

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>