Description of problem:
Got SIGABRT of multipathd when doing `show maps json` with 
`multipath -F; multipath -f`

```
#0  0x00007ffff691c1f7 in raise () from /lib64/libc.so.6
#1  0x00007ffff691d8e8 in abort () from /lib64/libc.so.6
#2  0x00007ffff695bf47 in __libc_message () from /lib64/libc.so.6
#3  0x00007ffff6963619 in _int_free () from /lib64/libc.so.6
#4  0x0000555555560038 in show_maps_json (r=0x7ffff7faad08, len=0x7ffff7faacf8, vecs=0x555555774280) at cli_handlers.c:157
#5  0x000055555555f4a7 in parse_cmd (cmd=cmd@entry=0x7fffe8001f70 "show maps json", reply=reply@entry=0x7ffff7faad08, len=len@entry=0x7ffff7faacf8, data=data@entry=0x555555774280)
    at cli.c:439
#6  0x0000555555559d29 in uxsock_trigger (str=<optimized out>, reply=reply@entry=0x7ffff7faad08, len=len@entry=0x7ffff7faacf8, is_root=is_root@entry=true, 
    trigger_data=trigger_data@entry=0x555555774280) at main.c:888
#7  0x000055555555e145 in uxsock_listen (uxsock_trigger=uxsock_trigger@entry=0x555555559cc0 <uxsock_trigger>, trigger_data=trigger_data@entry=0x555555774280) at uxlsnr.c:200
#8  0x000055555555a8b3 in uxlsnrloop (ap=0x555555774280) at main.c:1057
#9  0x00007ffff7bc6e25 in start_thread () from /lib64/libpthread.so.0
#10 0x00007ffff69df34d in clone () from /lib64/libc.so.6
```

Version-Release number of selected component (if applicable):
device-mapper-multipath-0.4.9-111.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. In a new terminal run
    for x in `seq 1 1000`; do sudo multipath -F ; sudo multipath -r; done
2. In a new terminal run
    for x in `seq 1 100`; do sudo  multipathd -k'show maps json'; done
3.

Actual results:
multipathd crash

Expected results:
multipathd dont crash

Additional info: