Don't relay alerts to peers before version negotiation

Fixes #1436

src/alert.cpp

@@ -127,6 +127,9 @@ bool CAlert::RelayTo(CNode* pnode) const
 {
     if (!IsInEffect())
         return false;
+    // don't relay to nodes which haven't sent their version message
+    if (pnode->nVersion == 0)
+        return false;
     // returns true if wasn't already contained in the set
     if (pnode->setKnown.insert(GetHash()).second)
     {