gsocks5proxy: Fix error reporting in authentication

set_auth_msg() was returning FALSE to indicate error, but all its
callers were expecting a negative return value to indicate error. This
was causing memory leaks for the GError, and errors to not be reported.

Coverity CID: 1325357

gio/gsocks5proxy.c

@@ -205,7 +205,7 @@ set_auth_msg (guint8	  *msg,
       g_set_error_literal (error, G_IO_ERROR, G_IO_ERROR_PROXY_FAILED,
 			   _("Username or password is too long for SOCKSv5 "
 			     "protocol."));
-      return FALSE;
+      return -1;
     }
 
   msg[len++] = SOCKS5_AUTH_VERSION;