fix [#35150] Crash when bmesh operation called from within a Panel draw()

accessing a bmesh from python would reallocate all customdata layers.

add an assert to BM_data_layer_free(), when its called unnecessarily since its reallocating all layers.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56396 954f8c5b-7b00-dc11-b283-0030488c597c