GTextEditor: Fixed bug in find_prev

Find_prev returned invalid when the contents of the file were equal to
the contents of the search box.

This is due to the checker walking on an empty character at the end of
a line.

Libraries/LibGUI/GTextEditor.cpp

@@ -1295,6 +1295,7 @@ GTextRange GTextEditor::find_prev(const StringView& needle, const GTextPosition&
         return {};
 
     GTextPosition position = start.is_valid() ? start : GTextPosition(0, 0);
+    position = prev_position_before(position);
     GTextPosition original_position = position;
 
     GTextPosition end_of_potential_match;