Prevent accessing unicode scalar element with incorrect index.

This change prevents a potential DocC crash by making sure that it does not access
an array element using an index that is outside of the bonds of the array.

`.index(after:` can crash in some cases if the passed index
belongs to the last element of the collection. with this change we ensure
that the passed index is not the last index.

Author: sofiaromorales

Sources/SwiftDocC/Utility/ValidatedURL.swift

@@ -178,8 +178,13 @@ private extension StringProtocol {
     /// Returns a percent encoded version of the string or the original string if it is already percent encoded.
     func addingPercentEncodingIfNeeded(withAllowedCharacters allowedCharacters: CharacterSet) -> String? {
         var needsPercentEncoding: Bool {
-            for (index, character) in unicodeScalars.indexed() where !allowedCharacters.contains(character) {
+            let unicodeScalars = unicodeScalars.filter { !allowedCharacters.contains($0) }
+            for (index, character) in unicodeScalars.indexed() {
                 if character == "%" {
+                    // There's not two characters after the "%". This "%" can't represent a percent encoded character.
+                    guard index != unicodeScalars.endIndex || index != unicodeScalars.index(before: index) else {
+                        return true
+                    }
                     // % isn't allowed in a URL fragment but it is also the escape character for percent encoding.
                     let firstFollowingIndex  = unicodeScalars.index(after: index)
                     let secondFollowingIndex = unicodeScalars.index(after: firstFollowingIndex)
@@ -188,7 +193,7 @@ private extension StringProtocol {
                         // There's not two characters after the "%". This "%" can't represent a percent encoded character.
                         return true
                     }
-                    // If either of the two following characters aren't hex digits, the "%" doesn't represent a
+                    // If either of the two following characters aren't hex digits, the "%" doesn't represent a percent encoded character.
                     return !Character(unicodeScalars[firstFollowingIndex]).isHexDigit
                         || !Character(unicodeScalars[secondFollowingIndex]).isHexDigit
 

Tests/SwiftDocCTests/Utility/ValidatedURLTests.swift

@@ -120,6 +120,13 @@ class ValidatedURLTests: XCTestCase {
             XCTAssertEqual(validatedWithHeading.components.path, expectedPath)
             XCTAssertEqual(validatedWithHeading.components.fragment, "Heading-Name")
         }
+        
+        for linkText in [
+            "url://com.example.test/new/docc=Whats%20New&version=DocC&Title=[Update]"
+        ] {
+            let validated = try XCTUnwrap(ValidatedURL(parsingAuthoredLink: linkText), "Failed to parse \(linkText.singleQuoted) as authored link")
+            XCTAssertEqual(validated.components.path.split(separator: "/").last!, "docc=Whats%20New&version=DocC&Title=[Update]")
+        }
     }
 
     func testEscapedFragment() throws {