Fix a potential crash situation when using IMAP voicemail

Fix a potential crash situation when using IMAP voicemail

If calling into VoiceMailMain when using IMAP storage, it was
possible to crash Asterisk by hanging up the phone when prompted
for a voicemail mailbox. This patch fixes the issue.

While it may appear that this patch is superficial, it allows code
execution to continue to the failure case just below the IMAP_STORAGE
code block where this patch has been applied

(closes issue #14473)
Reported by: dwpaul
Patches:
      voicemail_imap_crash_no_mailbox.patch uploaded by dwpaul (license 689)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@175590 65c4cc65-6c06-0410-ace0-fbb531ad65f3