[strategies][agnostic] fix bug in Graham-Andrew strategy:
last is used again after two calls to pop_back() which can invalidate
the reference;
patch: make last a true point rather than a reference
bug reported/patch suggested by David Zhao
Merge pull request #72 from mkaravel/fix/graham_andrew

[strategies][agnostic] fix bug in Graham-Andrew strategy:
Merge branch 'develop' into prepare