Tests: avoid uninitialized warnings in charset_gzip_static.t.

charset_gzip_static.t

diff --git a/charset_gzip_static.t b/charset_gzip_static.t
index 63a8733..a6a6e4a 100644
--- a/charset_gzip_static.t
+++ b/charset_gzip_static.t
@@ -90,7 +90,7 @@
 $t->write_file('t1.html.gz', '');
 
 my $in = 'X' x 99;
-my $out;
+my $out = '';
 
 eval {
 	require IO::Compress::Gzip;