Clean up some oversights from commit 0ec329c. This was triggered by fuzz testing finding
a memory leak with the original commit, see https://oss-fuzz.com/testcase-detail/4656781834452992
I was unable to replicate the fuzzing failure, but reviewing the nearby code showed
some likely problems.
In this commit,
- Fix UStack::pop() to not delete the popped element when a deleter function is present.
This was a bug, but because there were no stacks with deleters, was not causing trouble.
- Change RegexCompile::compileSet() to delete the set if it cannot be added to the internal
vector of sets. I suspect this is the cause of the fuzzing leak - 0ec329c changed the
behavior of UVector in the presence of errors.
- Changed RegexCompile::fSetStack to use an object deleter function. This fixes the
leak checking at the points new elements are pushed onto this stack.
