Improve Time

remove mrb_open NULL check for small test programs.  They are only for proof-on-concept test, not production code.  Adding precise checks are not needed for those code

guard mrb_open mem allocation and downstream usage

Class#superclass improvement

A rescue clause with no parameter list rescues only StandardErrors

fix an issue that access violation occurred on uninitialized irep block when running "make test" on MINGW.

solution: initialize memory allocated.