secureserver.pl: fix stunnel version parsing

- Allow the stunnel minor-version version part to be zero.

Prior to this change with the stunnel version scheme of <major>.<minor>
if either part was 0 then version parsing would fail, causing
secureserver.pl to fail with error "No stunnel", causing tests that use
the SSL protocol to be skipped. As a practical matter this bug can only
be caused by a minor-version part of 0, since the major-version part is
always greater than 0.

Closes https://github.com/curl/curl/pull/11722

tests/secureserver.pl

@@ -218,7 +218,7 @@ foreach my $veropt (('-version', '-V')) {
     }
     last if($ver_major);
 }
-if((!$ver_major) || (!$ver_minor)) {
+if((!$ver_major) || !defined($ver_minor)) {
     if(-x "$stunnel" && ! -d "$stunnel") {
         print "$ssltext Unknown stunnel version\n";
     }