Fix self-move-assignment UB in RE2::Set and FilteredRE2, add missing compiled_ guards

[XananasX7]

Summary

This PR fixes two classes of undefined behavior in RE2::Set and FilteredRE2:

Fix 1: Self-move-assignment undefined behavior (UB) in RE2::Set and FilteredRE2

Files: re2/set.cc, re2/filtered_re2.cc

The move assignment operators in both classes use a destroy-then-placement-new pattern:

RE2::Set& RE2::Set::operator=(Set&& other) {
  this->~Set();                        // destroys *this first
  (void) new (this) Set(std::move(other)); // then move-constructs from other
  return *this;
}

This is undefined behavior when this == &other (self-move-assignment). The destructor runs first, calling Decref() on internal Regexp* pointers and freeing resources. The subsequent move constructor then reads from the already-destroyed object — a use-after-free leading to potential double-free and heap corruption.

Self-move-assignment can occur through aliased references, for example:

std::vector<RE2::Set> v = ...;
v[i] = std::move(v[j]);  // undefined behavior if i == j

Fix: Add a self-assignment guard (if (this != &other)) before the destroy-and-reconstruct sequence, matching the standard idiomatic fix for this pattern.

Fix 2: Missing compiled_ guard in FilteredRE2::AllMatches() and FilteredRE2::AllPotentials()

File: re2/filtered_re2.cc

FilteredRE2::FirstMatch() correctly checks the compiled_ flag (line 100) and returns early with a DFATAL log if Compile() was not called. However, AllMatches() and AllPotentials() lack this guard entirely.

Calling either function before Compile() results in undefined behavior because the prefilter_tree_ has not been built — the same class of crash previously fixed for Filter.Match() in the Python bindings (issue #484).

Fix: Add identical compiled_ guards to AllMatches() and AllPotentials(), consistent with FirstMatch().

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Both fixes are minimal, targeted, and follow the existing style of the codebase.