resource_channels: Check if a passed in ID is NULL before checking its length

resource_channels: Check if a passed in ID is NULL before checking its length

Calling strlen on a NULL string is explosive. This patch checks whether or not
the passed in string is NULL or zero length before checking to see if the
string is too long.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@410274 65c4cc65-6c06-0410-ace0-fbb531ad65f3