LibRegex: Check backreference index before looking it up

If a backref happens after it's cleared, the slot may be cleared
already.

Libraries/LibRegex/RegexByteCode.cpp

@@ -585,6 +585,9 @@ ALWAYS_INLINE ExecutionResult OpCode_Compare::execute(MatchInput const& input, M
         }
         case CharacterCompareType::Reference: {
             auto reference_number = (size_t)m_bytecode->at(offset++);
+            if (input.match_index >= state.capture_group_matches.size())
+                return ExecutionResult::Failed_ExecuteLowPrioForks;
+
             auto& groups = state.capture_group_matches.at(input.match_index);
             if (groups.size() <= reference_number)
                 return ExecutionResult::Failed_ExecuteLowPrioForks;

Tests/LibRegex/Regex.cpp

@@ -741,6 +741,8 @@ TEST_CASE(ECMA262_match)
         // Tests nested lookahead with alternation - verifies proper save/restore stack cleanup
         { "a(?=.(?=c)|b)b"sv, "ab"sv, true },
         { "(?=)(?=\\d)"sv, "smart"sv, false },
+        // Backrefs are cleared after lookaheads, the indices should be checked before lookup.
+        { "(?!(b))\\1"sv, "a"sv, false },
     };
 
     for (auto& test : tests) {