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.

src/test/ssl/t/001_ssltests.pl

@@ -754,7 +754,7 @@ my $serialno = '\d+';
 
 if ($ENV{OPENSSL} ne '')
 {
-	$serialno = `$ENV{OPENSSL} x509 -serial -noout -in ssl/client.crt`;
+	my $serialstr = `$ENV{OPENSSL} x509 -serial -noout -in ssl/client.crt`;
 	if ($? == 0)
 	{
 		# OpenSSL prints serial numbers in hexadecimal and converting the serial
@@ -765,9 +765,9 @@ if ($ENV{OPENSSL} ne '')
 		{
 			no warnings qw(portable);
 
-			$serialno =~ s/^serial=//;
-			$serialno =~ s/\s+//g;
-			$serialno = hex($serialno);
+			$serialstr =~ s/^serial=//;
+			$serialstr =~ s/\s+//g;
+			$serialno = hex($serialstr);
 		}
 	}
 }