get_string: improved error checking with use of eval

From http://cvs.moodle.org/moodle/lib/moodlelib.php?r1=1.960.2.49&r2=1.960.2.50&pathrev=MOODLE_19_STABLE it appears that we were checking that eval functioned properly before continuing but we did not check the two parent language uses of eval. The attached patch adds the same check introduced above to the parent language section of the get_string function. Peace - Anthony