This patch gets rid of UB when verbosity exceeds the maximum logging value of `lvlVomit = 7` and
reaches invalid values (e.g. 8). This is actually triggered in functional tests.
There are too many occurrences to list, but here's one from the UBSAN log:
../src/libstore/gc.cc:610:5: runtime error: load of value 8, which is not a valid value for type 'Verbosity'