Fix uninitialized UpdateMask field

Initialized UpdateMask::_bits to NULL in all constructors.
UpdateMask(UpdateMask const& right) constructor sets the field count with SetCount() method before any field initialization. This means that SetCount() will call delete[] on the uninitialized _bits pointer field, leading to undefined behavior.
Merge pull request #10617 from jackpoz/updatemask_fix

Fix uninitialized UpdateMask field