ws: Fix use of g_string_erase() on arm7hl

It seems like we need this in order to pass build on Fedora 23 arm7hl.
Not sure what the deal is exactly, but it's really hard to debug,
and this is equivalent to the previous code, and makes it work.

Closes #3249

Reviewed-by: Dominik Perpeet <dperpeet@redhat.com>

src/ws/cockpitsshtransport.c

@@ -1109,7 +1109,7 @@ drain_error_buffer (CockpitSshTransport *self)
     {
       if (len > 0)
           g_printerr ("%s", data);
-      g_string_erase (self->errbuf, 0, -1);
+      g_string_erase (self->errbuf, 0, self->errbuf->len);
       pos = NULL;
     }
   else