Fix a memory leak on an error path

A leak of an SSL_SESSION object can occur when decoding a psk extension on
an error path when using TLSv1.3

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2843)
diff --git a/ssl/statem/extensions_srvr.c b/ssl/statem/extensions_srvr.c
index 83fb713..ab3ad46 100644
--- a/ssl/statem/extensions_srvr.c
+++ b/ssl/statem/extensions_srvr.c
@@ -796,6 +796,7 @@
     s->session = sess;
     return 1;
 err:
+    SSL_SESSION_free(sess);
     return 0;
 }