serial-dmm: Fix segfault when no device is used

di was initialized as NULL. If no device covered by this driver
is used, di remains NULL. This causes a segmentation fault when
calling clear_instances().

Check for di being NULL.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

hardware/serial-dmm/api.c

@@ -151,6 +151,10 @@ static int clear_instances(void)
 	struct dev_context *devc;
 	GSList *l;
 
+	/* di is not necessarily initialized */
+	if (!di)
+		return SR_OK;
+
 	if (!(drvc = di->priv))
 		return SR_OK;