wiki2markdown.pl: fixed a Perl warning.

Author: agentzh

wiki2markdown.pl

@@ -233,6 +233,9 @@ ($)
 #$s =~ s{^\[\[(\w+)\]\]}{my $name = $1; my $link = gen_link($name); warn "!!! $name"; defined($link) ? "[$name]($link)" : "[$name](http://wiki.nginx.org/$name)"}gsme;
 while ($s =~ s{^(\S[^\n]*?)?\[\[(\w+|".*?")\]\]}{
     my ($prefix, $name) = ($1, $2);
+    if (!defined $prefix) {
+        $prefix = '';
+    }
     my ($link, $modname) = gen_link($name);
     if (!defined $link) {
         warn "$name";