Fix sp no winners crash

[breznak]

Small PR that just avoids detected crash

• [x] adds test with small SP that would crash
  • [x] fix by avoiding out-of-bounds indexing in isWinner_ method.

Fixes #1390

[breznak]

Repo freeze. Moved to https://github.com/htm-community/nupic.cpp/pull/6 Closing here