When Prog::CompileSet() returns nullptr (e.g., DFA memory
exhaustion), the internal state of Set becomes inconsistent:
compiled_ is already set to true but prog_ is nullptr.
A subsequent call to Set::Match() passes the !compiled_ check
and dereferences the null prog_.

Fix: set compiled_ based on whether compilation actually succeeded,
rather than unconditionally before it. Similarly, defer elem_.clear()
so that state remains valid for retry on failure.

This restores the invariant: compiled_ implies prog_ != nullptr.

Also add a regression test using max_mem(1) to guarantee
compilation failure.