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>

1 files changed, 4 insertions, 1 deletions

diff --git a/super1.c b/super1.c
index 7bee0265..70aa1d3d 100644
--- a/super1.c
+++ b/super1.c
@@ -2717,7 +2717,10 @@ static int write_bitmap1(struct supertype *st, int fd, enum bitmap_update update
 
 	init_afd(&afd, fd);
 
-	locate_bitmap1(st, fd, 0);
+	if (locate_bitmap1(st, fd, 0) < 0) {
+		pr_err("Error: Invalid bitmap\n");
+		return -EINVAL;
+	}
 
 	if (posix_memalign(&buf, 4096, 4096))
 		return -ENOMEM;