zdtm/s390x_regs_check: Fix compiler warning

When running 'make zdtm' on s390x it fails on RHEL7 with:

 make[3]: Leaving directory `/tmp/criu/test/zdtm/lib'
  CC        s390x_regs_check.o
 s390x_regs_check.c: In function "util_hexdump_grp":
 s390x_regs_check.c:214:7: error: "ptr" may be used uninitialized
 in this function [-Werror=maybe-uninitialized]
    ptr += sprintf(ptr, "%02x", buf[i]);

Fix this and assign ptr from the beginning to help gcc.
Reported-by: Adrian Reber <adrian@lisas.de>
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Acked-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/static/s390x_regs_check.c

@@ -198,8 +198,8 @@ struct reg_set *reg_set_vec[] = {
 void util_hexdump_grp(const char *tag, const void *data, int grp,
 		      int count, int indent)
 {
+	char str[1024], *ptr = str;
 	const char *buf = data;
-	char str[1024], *ptr;
 	int i, first = 1;
 
 	for (i = 0; i < count; i++) {