Avoid passing -1 as an fd to uxsel_set().

Avoid passing -1 as an fd to uxsel_set().

I'd missed out an if statement in the Unix proxy stderr code
introduced by commit 297efff30, causing ret->cmd_err to be passed to
uxsel_set even when it was -1 (which happened in the non-GUI tools).
Unfortunately, putting a negative fd into the uxsel tree has really
bad effects, because the first_fd / next_fd interface returns a
negative number to signal end-of-list - and since the uxsel tree is
sorted by fd, that happens _immediately_.

Added the missing if statement, and also an assertion to make sure we
never pass -1 to uxsel_set by mistake again!