Stop at end of search string in TwoWaySearcher (#631)

When searching for a substring that doesn't exist, it was possible
for TwoWaySearcher to advance beyond the end of the search string,
causing a crash. This change adds a `limitedBy:` parameter to that
index movement, avoiding the invalid movement.

Fixes rdar://105154010

Author: natecook1000

Sources/_StringProcessing/Algorithms/Searchers/TwoWaySearcher.swift

@@ -133,7 +133,10 @@ extension TwoWaySearcher: CollectionSearcher {
       searched.formIndex(before: &lIndex)
 
       if pattern[i] != searched[lIndex] {
-        searched.formIndex(&state.criticalIndex, offsetBy: period)
+        _ = searched.formIndex(
+          &state.criticalIndex,
+          offsetBy: period,
+          limitedBy: searched.endIndex)
         if periodIsExact { state.memory = (pattern.count - period, end) }
         return nil
       }

Tests/RegexTests/AlgorithmsTests.swift

@@ -174,6 +174,16 @@ class AlgorithmTests: XCTestCase {
     expectRanges("ababacabababa", "abababa", [6..<13])
     expectRanges("ababacabababa", "aba", [0..<3, 6..<9, 10..<13])
   }
+  
+  // rdar://105154010
+  func testFirstRangeMissingCrash() {
+    let str = "%2$@ %#@AROUND_TIME@"
+    let target = "%@"
+    XCTAssertNil(str.firstRange(of: target))
+    XCTAssertNil(str.dropFirst().dropLast().firstRange(of: target))
+    XCTAssertNil(str.dropFirst().dropLast().firstRange(of: target[...]))
+    XCTAssertNil(str.firstRange(of: target[...]))
+  }
 
   func testRegexSplit() {
     func expectSplit(