super1: fix Floating point exception

super1: fix Floating point exception

write_bitmap1 didn't check return value of locate_bitmap1, which will
operate bitmap area under invalid bitmap info.

mdadm core dumped when doing below steps:
```
node1 # mdadm -C /dev/md0 -b none -e 1.2 -n 2 -l mirror /dev/sda /dev/sdb
node1 # mdadm -Ss
node1 # mdadm -A -U home-cluster --home-cluster=abc /dev/md0 /dev/sda /dev/sdb
Floating point exception (core dumped)
```

Signed-off-by: Zhao Heming <heming.zhao@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>