Plug up some holes with lib_cvRegisterVar

Fixes few issues found with the Lua version of CV_RegisterVar and adds measurements to prevent such things.

• Error if a consvar is set to an empty string
• Error if a consvar is not given a defaultvalue (fixes #441 (closed))

added Bug Lua labels

changed milestone to %2.2.11

merged

mentioned in commit 7f913b5a