Fix crash in RESOLVE/extend/only

Ensure that the internal helper function `Expand_Frame` properly terminates the frame's word and value series, even when _not_ copying from the original frame.

The `/extend` handling of `RESOLVE` is currently the only user of non-copying `Expand_Frame`. Without proper termination, the code in `RESOLVE` makes invalid memory accesses at (or past) the frame's tail, causing crashes.

This fixes [CureCode issue #2017](http://issue.cc/r3/2017).