Was incorrectly testing for a vertex in a set with BLI_ghash_lookup rather than BLI_ghash_haskey; the key in this case is always null so the test failed. This could leave the PBVH in an inconsistent state, since the top-level map of BMesh vertices to PBVH nodes would indicate the vertex was in a node, but that node wouldn't actually have any faces using the vertex. That inconsistent state would eventually lead to a crash in pbvh_bmesh_vert_remove(). Fixes http://projects.blender.org/tracker/?func=detail&atid=498&aid=34370&group_id=9