SSL: missing free calls in $ssl_client_s_dn and $ssl_client_i_dn.

If X509_get_issuer_name() or X509_get_subject_name() returned NULL,
this could lead to a certificate reference leak.  It cannot happen
in practice though, since each function returns an internal pointer
to a mandatory subfield of the certificate successfully decoded by
d2i_X509() during certificate message processing (closes #1751).

src/event/ngx_event_openssl.c

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index bee264c..7be4fb4 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -4622,6 +4622,7 @@
 
     name = X509_get_subject_name(cert);
     if (name == NULL) {
+        X509_free(cert);
         return NGX_ERROR;
     }
 
@@ -4673,6 +4674,7 @@
 
     name = X509_get_issuer_name(cert);
     if (name == NULL) {
+        X509_free(cert);
         return NGX_ERROR;
     }