bitvec: make bitvec_free() safe against NULL

Change-Id: Ifafb65e9d0adc286e16104274db440f38a86d800
Related: CID#208181, CID#208179
diff --git a/src/bitvec.c b/src/bitvec.c
index ef8621c..5130705 100644
--- a/src/bitvec.c
+++ b/src/bitvec.c
@@ -418,6 +418,8 @@
  *  \param[in] bit vector to free */
 void bitvec_free(struct bitvec *bv)
 {
+	if (bv == NULL)
+		return;
 	talloc_free(bv->data);
 	talloc_free(bv);
 }