[net] Avoid possibility of NULL pointer dereference in MarkBlockAsInFlight(...)

In the case that the branch ...

    if (itInFlight != mapBlocksInFlight.end() && itInFlight->second.first == nodeid) {

... is taken, there was prior to this commit an implicit assumption that
MarkBlockAsInFlight(...) was being called with its fifth and optional
argument (pit) being present (and non-NULL).

src/net_processing.cpp

@@ -338,7 +338,9 @@ bool MarkBlockAsInFlight(NodeId nodeid, const uint256& hash, const Consensus::Pa
     // Short-circuit most stuff in case its from the same node
     map<uint256, pair<NodeId, list<QueuedBlock>::iterator> >::iterator itInFlight = mapBlocksInFlight.find(hash);
     if (itInFlight != mapBlocksInFlight.end() && itInFlight->second.first == nodeid) {
+        if (pit) {
             *pit = &itInFlight->second.second;
+        }
         return false;
     }