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.