app_voicemail: Don't split mailbox options on comma

Because the per-mailbox options are the last thing on a line, don't look
for or stomp on any subsequent commas.

ASTERISK-27935 #close
Reported by: Sébastien Duthil

Change-Id: I07b2eb4a33c303d0c7114d5b906f8c067c60a153