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>