vweb: fix a leak

vlib/strings/builder.c.v

@@ -115,6 +115,7 @@ pub fn (mut b Builder) str() string {
 	b.buf << `\0`
 	s := string(b.buf,b.len)
 	bis := b.initial_size
+	//free(b.buf.data)
 	b.buf = []byte{cap: bis}
 	b.len = 0
 	return s