BUG/MINOR: ssl: Missing calloc return value check in ssl_init_single_engine

A memory allocation failure happening during ssl_init_single_engine
would have resulted in a crash. This function is only called during
init.

It was raised in GitHub issue #1233.
It could be backported to all stable branches.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index ec271c9..f596a83 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -636,6 +636,8 @@
 	}
 
 	el = calloc(1, sizeof(*el));
+	if (!el)
+		goto fail_alloc;
 	el->e = engine;
 	LIST_INSERT(&openssl_engines, &el->list);
 	nb_engines++;
@@ -643,6 +645,7 @@
 		global.ssl_used_async_engines = nb_engines;
 	return 0;
 
+fail_alloc:
 fail_set_method:
 	/* release the functional reference from ENGINE_init() */
 	ENGINE_finish(engine);