Add casts for unavoidable signed/unsigned comparisons

At these code sites, it is preferable to cast rather than change
a variable's type.

src/protocol.cpp

@@ -128,7 +128,7 @@ bool operator<(const CInv& a, const CInv& b)
 
 bool CInv::IsKnownType() const
 {
-    return (type >= 1 && type < ARRAYLEN(ppszTypeName));
+    return (type >= 1 && type < (int)ARRAYLEN(ppszTypeName));
 }
 
 const char* CInv::GetCommand() const