Fix ending the dynamic extent in Text_interface::interpret_markup

Fix ending the dynamic extent in Text_interface::interpret_markup

When using cyclic references under guile-2.0 lilypond crashes with
a segmentation fault:

  $ LANG=C out/bin/lilypond input/regression/markup-cyclic-reference.ly
  GNU LilyPond 2.19.51
  Processing `input/regression/markup-cyclic-reference.ly'
  Parsing...
  Finding the ideal number of pages...
  Fitting music on 1 page...
  Drawing systems...
  Layout output to `/tmp/lilypond-wkUlSF'...
  Converting to `markup-cyclic-reference.pdf'...
  Deleting `/tmp/lilypond-wkUlSF'...
  Segmentation fault

This happens because in Text_interface::interpret_markup the dynamic
extent is not ended properly in the exit path for the case when the
markup depth exceeds the max_depth.