Fix NULL dereference in super_by_fd

Fix NULL dereference in super_by_fd

When we create 100 partitions (major is 259 not 254) in a raid device,
mdadm may coredump:

Core was generated by `/usr/sbin/mdadm --detail --export /dev/md1p7'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  __strlen_avx2_rtm () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:74
74		VPCMPEQ	(%rdi), %ymm0, %ymm1
(gdb) bt
#0  __strlen_avx2_rtm () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:74
#1  0x00007fbb9a7e4139 in __strcpy_chk (dest=dest@entry=0x55d55d6a13ac "", src=0x0, destlen=destlen@entry=32) at strcpy_chk.c:28
#2  0x000055d55ba1766d in strcpy (__src=<optimized out>, __dest=0x55d55d6a13ac "") at /usr/include/bits/string_fortified.h:79
#3  super_by_fd (fd=fd@entry=3, subarrayp=subarrayp@entry=0x7fff44dfcc48) at util.c:1289
#4  0x000055d55ba273a6 in Detail (dev=0x7fff44dfef0b "/dev/md1p7", c=0x7fff44dfe440) at Detail.c:101
#5  0x000055d55ba0de61 in misc_list (c=<optimized out>, ss=<optimized out>, dump_directory=<optimized out>, ident=<optimized out>, devlist=<optimized out>) at mdadm.c:1959
#6  main (argc=<optimized out>, argv=<optimized out>) at mdadm.c:1629

The direct cause is fd2devnm returning NULL, so add a check.

Signed-off-by: Li Xiao Keng <lixiaokeng@huawei.com>
Signed-off-by: Wu Guang Hao <wuguanghao3@huawei.com>
Acked-by: Coly Li <colyli@suse.de>
Acked-by: Coly Li <colyli@suse.de <mailto:colyli@suse.de>>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>