hashmap: change return value type of ext2fs_hashmap_add()
In ext2fs_hashmap_add(), new entry is allocated by calling
malloc(). If malloc() return NULL, it will cause a
segmentation fault problem.
Here, we change return value type of ext2fs_hashmap_add()
from void to int. If allocating new entry fails, we will
return -1, and the callers should also verify the return
value of ext2fs_hashmap_add().