op_free(): don't assert op_private ok when erred

op_free(): don't assert op_private ok when erred

[perl #126258]

op_free includes an assert to make sure each op's op_private field
only has the flags set that are expected for that op. It's a thing I added
at the same time I added the regen/op_private mechanism, and is more a
general "make sure people are only setting the flags we know about" test.

However, if the op tree is being freed after a compilation error, some
op's flags may be in an inconsistent state; so skip the assert in that case.

e.g.

    perl -e 'grep$0,0}'