logging_vty: set osmo_stderr_target to NULL on "no log stderr"

We cannot keep a reference to the old log_target around if we
just destroyed/deleted it.

Change-Id: I98a801cf83347a4422534d020d5ed5e2d1eb5482
diff --git a/src/vty/logging_vty.c b/src/vty/logging_vty.c
index 0e1782a..bd513e3 100644
--- a/src/vty/logging_vty.c
+++ b/src/vty/logging_vty.c
@@ -33,6 +33,7 @@
 #include <osmocom/core/strrb.h>
 #include <osmocom/core/loggingrb.h>
 #include <osmocom/core/gsmtap.h>
+#include <osmocom/core/application.h>
 
 #include <osmocom/vty/command.h>
 #include <osmocom/vty/buffer.h>
@@ -797,6 +798,7 @@
 	}
 
 	log_target_destroy(tgt);
+	osmo_stderr_target = NULL;
 
 	RET_WITH_UNLOCK(CMD_SUCCESS);
 }