Fix for collapse-edges crash in dyntopo

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

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54827 954f8c5b-7b00-dc11-b283-0030488c597c
Hidding the append: already linked message if G.debug is false. This gets rid of some noise in the console for BGE games that run into this case a lot.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54828 954f8c5b-7b00-dc11-b283-0030488c597c