rtpengine: fix segfault when bad mi params

Default root pointer to NULL in order to avoid segfaults due to bad mi params
(e.g string instead of int). The segfaults happened because in the above cases,
free_mi_tree(root) tries to free a random, invalid address.

modules/rtpengine/rtpengine.c

@@ -854,7 +854,7 @@ static struct mi_root* mi_enable_rtp_proxy(struct mi_root *cmd_tree,
 	struct mi_node *node, *crt_node;
 	struct rtpp_set *rtpp_list;
 	struct rtpp_node *crt_rtpp, *found_rtpp;
-	struct mi_root *root;
+	struct mi_root *root = NULL;
 	struct mi_attr *attr;
 	unsigned int enable;
 	int found, found_rtpp_disabled;
@@ -1099,7 +1099,7 @@ static struct mi_root* mi_show_rtp_proxy(struct mi_root* cmd_tree,
 												void* param)
 {
 	struct mi_node *node;
-	struct mi_root *root;
+	struct mi_root *root = NULL;
 	struct rtpp_set *rtpp_list;
 	struct rtpp_node *crt_rtpp;
 	int found;
@@ -1190,7 +1190,7 @@ static struct mi_root* mi_ping_rtp_proxy(struct mi_root* cmd_tree,
 {
 	struct mi_node *node, *crt_node;
 	struct mi_attr *attr;
-	struct mi_root *root;
+	struct mi_root *root = NULL;
 	struct rtpp_set *rtpp_list;
 	struct rtpp_node *crt_rtpp, *found_rtpp;
 	int found, found_rtpp_disabled;