Catch repeated calls to osmo_init_logging()

If an application calls osmo_init_logging() multiple times, let's
bail out in a safe way without corrupting the state + returning an
error.

Change-Id: Icf337a430fb367bbca48a1b02822a2cb3b644e5f
diff --git a/src/application.c b/src/application.c
index ee22639..538ac45 100644
--- a/src/application.c
+++ b/src/application.c
@@ -112,6 +112,12 @@
  */
 int osmo_init_logging(const struct log_info *log_info)
 {
+	static int logging_initialized = 0;
+
+	if (logging_initialized)
+		return -EEXIST;
+
+	logging_initialized = 1;
 	log_init(log_info, NULL);
 	osmo_stderr_target = log_target_create_stderr();
 	if (!osmo_stderr_target)