BUG/MINOR: ssl: increment issuer refcount if in chain

When using the OCSP response, if the issuer of the response is in
the certificate chain, its address will be stored in ckch->ocsp_issuer.
However, since the ocsp_issuer could be filled by a separate file, this
pointer is free'd. The refcount of the X509 need to be incremented to
avoid a double free if we free the ocsp_issuer AND the chain.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 4ff051b..db9621b 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -3468,6 +3468,7 @@
 			issuer = sk_X509_value(ckch->chain, i);
 			if (X509_check_issued(issuer, ckch->cert) == X509_V_OK) {
 				ckch->ocsp_issuer = issuer;
+				X509_up_ref(ckch->ocsp_issuer);
 				break;
 			} else
 				issuer = NULL;