LibGUI: Fix crash when getting text of selected line after a line delete

Author: https://github.com/ngc6302h
Commit: b7d726e226
Pull-request: https://github.com/SerenityOS/serenity/pull/5879
Issue: https://github.com/SerenityOS/serenity/issues/5875
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibGUI/TextDocument.cpp

@@ -343,7 +343,7 @@ String TextDocument::text() const
 
 String TextDocument::text_in_range(const TextRange& a_range) const
 {
-    if (is_empty())
+    if (is_empty() || line_count() < a_range.end().line() - a_range.start().line() || line(a_range.start().line()).is_empty())
         return String("");
     auto range = a_range.normalized();