Revert error handling in Mat_VarFree
Revert error handling in Mat_VarFree

* Mat_VarFree should not fail with Mat_Critical
* Fix potential memory leaks by early returns

MaartenBent/matio,tbeu/matio,MaartenBent/matio,MaartenBent/matio,tbeu/matio,tbeu/matio,tbeu/matio,MaartenBent/matio