Realtime: Fix a bug that caused realtime destroy command to crash

Also has could affect with anything that goes through ast_destroy_realtime.
If a CLI user used the command 'realtime destroy <family>' with only a single
column/value pair, Asterisk would crash when trying to create a variable list
from a NULL value.

ASTERISK-24231 #close
Reported by: Niklas Larsson
Review: https://reviewboard.asterisk.org/r/3985/



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@422984 f38db490-d61c-443f-a65b-d21fe96a405b