Fix crash for relative links

Sources/Generator/Content/Element.swift

@@ -431,7 +431,7 @@ extension Element {
 
         // Find the common elements of the path, which can be discarded
         var index = 0
-        while pageParts[index] == ownParts[index] {
+        while index < pageParts.count && index < ownParts.count && pageParts[index] == ownParts[index] {
             index += 1
         }
         // The relative path needs to go down to the first common folder,

Sources/Generator/Generators/PageContentGenerator.swift

@@ -278,7 +278,10 @@ struct PageContentGenerator {
                     results.warning("Invalid external link definition", source: page.path)
                     return nil
                 }
-                let url = parts[0].trimmed
+                guard let url = parts[0].trimmed.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed) else {
+                    results.warning("Invalid external link \(parts[0].trimmed)", source: page.path)
+                    return nil
+                }
                 let title = parts[1].trimmed
 
                 return (url, title)