Fix strncmp(...) length when looking for "edges"

Any tree contain edge data would fail to read. As the
strncmp(...) to look for "edges" was passing a length
of 4, this would also match "edge_data"! Use a length
of 5 instead.

Change-Id: I3ad38600edbadb4a8cd89f5bb4f448cff64f0f23
Merge topic 'fix-vtktreereader' into master

ae9aeb16 Fix strncmp(...) length when looking for "edges"