sip presence: fix uninitialized class member

Set default value of mutex_nesting_level_ to 0.

Coverity CID # 1291689

Change-Id: Ib794fe5a90679669c837e2ad8064998caae01f42
Tuleap: #909

src/sip/sippresence.h

@@ -237,7 +237,7 @@ class SIPPresence {
         std::list< PresSubClient *> sub_client_list_; /**< Subcribed buddy list.*/
 
         std::recursive_mutex mutex_;
-        unsigned         mutex_nesting_level_;
+        unsigned         mutex_nesting_level_ {0};
         pj_caching_pool  cp_;
         pj_pool_t       *pool_;
 };