BMesh: collapse_uvs - avoid clearing and flagging for every cd-layer
Make memory checker happy about FileGlobal

FileGlobal structure in write_global() didn't initialize
the while build_hash property and only placed there null
terminated meaningful content, leaving trailing uninitialized.

This made valgrind complain about using uninitialized memory
later in undo stack.
Fix T38312: Crash when using snap transform in edit mode

Issue was caused by uninitialized boolean flag.