Bug#739738: patch for this bug

[Alexander Inyukhin]

This patch fixes the bug.
Please apply.

--- a/lib/Mail/SpamAssassin/Util.pm
+++ b/lib/Mail/SpamAssassin/Util.pm
@@ -285,7 +285,7 @@
   my $r = ref $_[0];
   if (!$r) {
     no re 'taint';  # override a  "use re 'taint'"  from outer scope
-    return if !defined $_[0];
+    return undef if !defined $_[0];
     local($1); # avoid Perl taint bug: tainted global $1 propagates taintedness
     $_[0] =~ /^(.*)\z/s;
     return $1;