device: silent compiler warning

Fix the following warning:

 src/devices/nm-device.c: In function ‘activation_source_schedule’:
 src/devices/nm-device.c:4995:9: error: ‘source_func’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   new_id = g_idle_add (source_func, self);
  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1 files changed, 1 insertions, 1 deletions

diff --git a/src/devices/nm-device.c b/src/devices/nm-device.c
index bb624adf1b..010172dc06 100644
--- a/src/devices/nm-device.c
+++ b/src/devices/nm-device.c
@@ -4975,7 +4975,7 @@ static void
 activation_source_schedule (NMDevice *self, ActivationHandleFunc func, int addr_family)
 {
 	ActivationHandleData *act_data;
-	GSourceFunc source_func;
+	GSourceFunc source_func = NULL;
 	guint new_id = 0;
 
 	act_data = activation_source_get_by_family (self, addr_family, &source_func);