Reset skinny vmexten on reload.

Make skinny reset vmexten '\0' on reload to ensure that
it is set to '\0' if the appropriate item is removed/commented in 
skinny.conf. part of ASTERISK-21037 

Reported by: snuffy
Tested by: snuffy, myself
Patches: 
    part of immed_dial_fix.diff uploaded by snuffy (license 5024)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@380926 f38db490-d61c-443f-a65b-d21fe96a405b