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>

kernel/auditsc.c

@@ -1000,7 +1000,7 @@ static void audit_log_execve_info(struct audit_context *context,
 	long len_rem;
 	long len_full;
 	long len_buf;
-	long len_abuf;
+	long len_abuf = 0;
 	long len_tmp;
 	bool require_data;
 	bool encode;