Fix null pointer for incremental in mdadm

Fix null pointer for incremental in mdadm

when we excute mdadm --assemble, udev-md-raid-assembly.rules is triggered.
Then we stop array, we found an coredump for mdadm --incremental.func
stack are as follows:

#0  enough (level=10, raid_disks=4, layout=258, clean=1,
    avail=avail@entry=0x0) at util.c:555
#1  0x0000562170c26965 in Incremental (devlist=<optimized out>,
    c=<optimized out>, st=0x5621729b6dc0) at Incremental.c:514
#2  0x0000562170bfb6ff in main (argc=<optimized out>,
    argv=<optimized out>) at mdadm.c:1762

func enough() use array avail,avail allocate space in func count_active,
it may not alloc space, causing a coredump.We fix this coredump.

Signed-off-by: Guanqin Miao <miaoguanqin@huawei.com>
Signed-off-by: lixiaokeng <lixiaokeng@huawei.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>