UB causing segfaults in `nft::is_in_lang()`

[Adda0]

There is a UB in nft::is_in_lang() that causes segfaults, but appears very rarely. The bug can be seen in the following test: #601 (comment).

Might be related to another UB appearing in mata::nft::union_nondet():

### mata::nft::union_nondet() minimal
failing minimal scenario
mata/tests/nft/nft.cc:2730
...............................................................................
mata/tests/nft/nft.cc:2730: FAILED:
{Unknown expression after the reported line}
due to a fatal error condition:
SIGSEGV - Segmentation violation signal

Both UBs need to be fixed.

[koniksedy]

This bug is fixed by #550. It was caused by not updating levels inside union_nondet_with().