added asserts to check m_addr_known when it's used

src/net.h

@@ -884,6 +884,7 @@ public:
 
     void AddAddressKnown(const CAddress& _addr)
     {
+        assert(m_addr_known);
         m_addr_known->insert(_addr.GetKey());
     }
 
@@ -892,6 +893,7 @@ public:
         // Known checking here is only to save space from duplicates.
         // SendMessages will filter it again for knowns that were added
         // after addresses were pushed.
+        assert(m_addr_known);
         if (_addr.IsValid() && !m_addr_known->contains(_addr.GetKey())) {
             if (vAddrToSend.size() >= MAX_ADDR_TO_SEND) {
                 vAddrToSend[insecure_rand.randrange(vAddrToSend.size())] = _addr;