[WWcvs] pg: Avoid problem with undefined value being passed to protectHTML.

[dpvc v. a. <we...@ma...>]

Log Message:
-----------
Avoid problem with undefined value being passed to protectHTML.

Modified Files:
--------------
    pg/lib/Value:
        AnswerChecker.pm

Revision Data
-------------
Index: AnswerChecker.pm
===================================================================
RCS file: /webwork/cvs/system/pg/lib/Value/AnswerChecker.pm,v
retrieving revision 1.52
retrieving revision 1.53
diff -Llib/Value/AnswerChecker.pm -Llib/Value/AnswerChecker.pm -u -r1.52 -r1.53
--- lib/Value/AnswerChecker.pm
+++ lib/Value/AnswerChecker.pm
@@ -480,6 +480,7 @@
 #
 sub protectHTML {
     my $string = shift;
+    return unless defined($string);
     return $string if eval ('$main::displayMode') eq 'TeX';
     $string =~ s/&/\&/g;
     $string =~ s/</\&lt;/g;