Correctly reset the dialplan priority.

Correctly reset the dialplan priority.

When the stack frame is allocated, we save the address to which we should
return, when the Gosub returns.  However, if we just want to restore the
priority, then we need to subtract 1 before setting it.  Otherwise, when
a Gosub goes to a nonexistent address, it will skip a priority in the
dialplan.  This is because when we return from an application, the PBX
increments the priority for us.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@357416 65c4cc65-6c06-0410-ace0-fbb531ad65f3