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 part 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.
Merge pull request #115 from earl/fix-2017-resolve

Fix crash in RESOLVE/extend/only