LibMarkdown: Fix crash when rendering unfinished link

This commit fixes a crash where the Markdown preview attempts to render
`[](`, i.e. an unfinished link. This can happen when using TextEditor
with Markdown preview mode enabled.

Co-authored-by: Liz Cody <[email protected]>

Userland/Libraries/LibMarkdown/Text.cpp

@@ -668,7 +668,8 @@ NonnullOwnPtr<Text::Node> Text::parse_link(Vector<Token>::ConstIterator& tokens)
 
     StringBuilder address;
 
-    bool is_escaped = *(tokens + 1) == "<"sv;
+    auto next = tokens + 1;
+    bool is_escaped = !next.is_end() && *next == "<"sv;
     // Don't add the angle bracket to the address.
     if (is_escaped)
         tokens++;