s390x: silence warning from GCC on uninitialized values

As best I can tell, this is a false positive.

  [aliguori@ccnode4 qemu-s390]$ make
    CC    s390x-softmmu/target-s390x/helper.o
  /home/aliguori/git/qemu/target-s390x/helper.c: In function ‘do_interrupt’:
  /home/aliguori/git/qemu/target-s390x/helper.c:673:17: error: ‘addr’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  /home/aliguori/git/qemu/target-s390x/helper.c:620:20: note: ‘addr’ was declared here
  /home/aliguori/git/qemu/target-s390x/helper.c:673:17: error: ‘mask’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  /home/aliguori/git/qemu/target-s390x/helper.c:620:14: note: ‘mask’ was declared here
  cc1: all warnings being treated as errors
  make[1]: *** [target-s390x/helper.o] Error 1
  make: *** [subdir-s390x-softmmu] Error 2

Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
