Assert CPubKey::ValidLength to the pubkey's header-relevent size

Previously this was an inline test where the specificity was probably judged
overly specific. As a class method it makes sense to maintain consistency.

And replace some magic values with their constant equivalents.

src/keystore.cpp

@@ -131,7 +131,7 @@ static bool ExtractPubKey(const CScript &dest, CPubKey& pubKeyOut)
     CScript::const_iterator pc = dest.begin();
     opcodetype opcode;
     std::vector<unsigned char> vch;
-    if (!dest.GetOp(pc, opcode, vch) || vch.size() < 33 || vch.size() > 65)
+    if (!dest.GetOp(pc, opcode, vch) || !CPubKey::ValidSize(vch))
         return false;
     pubKeyOut = CPubKey(vch);
     if (!pubKeyOut.IsFullyValid())