Fix thinko in commit 7d129ba54.

Fix thinko in commit 7d129ba54.

The revised logic in 001_ssltests.pl would fail if openssl
doesn't work or if Perl is a 32-bit build, because it had
already overwritten $serialno with something inappropriate
to use in the eventual match.  We could go back to the
previous code layout, but it seems best to introduce a
separate variable for the output of openssl.

Per failure on buildfarm member mamba, which has a 32-bit Perl.