lib: fix offset in tst_resm_hexd

Commit f72ca5b9c484 "lib: add tst_res_hexd for newlib"
broke tst_resm_hexd when I moved "offset = strlen(tmesg)"
before macro that expands varargs.

Fix it by moving offset initialisation after EXPAND_VAR_ARGS.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/lib/tst_res.c b/lib/tst_res.c
index 261dec0..61daaeb 100644
--- a/lib/tst_res.c
+++ b/lib/tst_res.c
@@ -509,7 +509,7 @@
 	char tmesg[USERMESG];
 	static const size_t symb_num	= 2; /* xx */
 	static const size_t size_max	= 16;
-	size_t offset = strlen(tmesg);
+	size_t offset;
 	size_t i;
 	char *pmesg = tmesg;
 	tst_res_func_t res_func;
@@ -520,6 +520,7 @@
 		res_func = tst_res__;
 
 	EXPAND_VAR_ARGS(tmesg, arg_fmt, USERMESG);
+	offset = strlen(tmesg);
 
 	if (size > size_max || size == 0 ||
 		(offset + size * (symb_num + 1)) >= USERMESG)