Another correction to svn rev56509

Initial fix was not fully working, because faulty
edge shared the same address as screen area. This
lead to screen freeing issues -- double free (which
was supressed by guarded allocation actually) but
also freed memory access when iterating via edges
to free them.

Solved by a small hack which removes edges with bad
vertices from edges list. This prevents double-free
and freed memory access cased by corrupted files we're
currently fixing. In other cases this tweak is likely
be harmless -- in worst case scenario it'll lead to
small memory leak, which is not as much lethal as
freed memory access.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56511 954f8c5b-7b00-dc11-b283-0030488c597c