1.0.6.23: fix a source location tracking problem in sb-cover

1.0.6.23: fix a source location tracking problem in sb-cover

         * The logic for selecting the innermost form that contains
           a given character in the source file did not work correctly in
           cases where an exact location could not be found for some source
           path. This should mainly happen when reader macros are involved.
         * Instead of sorting the source paths by the length of the path,
           sort them by the length of the character range of the source
           location.