search.Beam: Fix invalid indexing (#633)

If none of the states of the beam have valid moves, it was attempted to
index into the vector of scored valid moves anyway, which results in
invalid memory access.

Fix this by first checking that there valid moves before attempting to
index into them.

thinc/extra/search.pyx

@@ -193,13 +193,14 @@ cdef class Beam:
  entry.first = s.score + scores[i][j]
  entry.second = move_id + j
  entries.push_back(entry)
- cdef double max_ = entries[0].first
- cdef double Z = 0.
- cdef double cutoff = 0.0
+ cdef double max_, Z, cutoff
  if self.min_density == 0.0:
  for i in range(entries.size()):
  q.push(entries[i])
- else:
+ elif not entries.empty():
+ max_ = entries[0].first
+ Z = 0.
+ cutoff = 0.
  # Softmax into probabilities, so we can prune
  for i in range(entries.size()):
  if entries[i].first > max_: