Fix a possible integer overflow in Ftp::Gateway (squid-cache#1647)

A static analysis tool has discovered that const int csize,
might have overflowed before being passed to writeReplyBody().

src/clients/FtpGateway.cc

@@ -1000,7 +1000,7 @@ Ftp::Gateway::processReplyBody()
         parseListing();
         maybeReadVirginBody();
         return;
-    } else if (const int csize = data.readBuf->contentSize()) {
+    } else if (const auto csize = data.readBuf->contentSize()) {
         writeReplyBody(data.readBuf->content(), csize);
         debugs(9, 5, "consuming " << csize << " bytes of readBuf");
         data.readBuf->consume(csize);