Fix #35177: Press P Crashes Blender After Fracture of Cube

Root of the issue was fixed by Brecht in svn rev56441.
This change only prevents crash of files created in
blender before that fix, and also gives more useable
information about what's wrong (apparently, assert
here was doing nothing).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56468 954f8c5b-7b00-dc11-b283-0030488c597c