Add check for valid keys in `importprivkey`

The base58 armoring was checked, but not the resulting private key,
which could be out of range. Fix this by adding a check.

src/rpcdump.cpp

@@ -101,9 +101,11 @@ Value importprivkey(const Array& params, bool fHelp)
     CBitcoinSecret vchSecret;
     bool fGood = vchSecret.SetString(strSecret);
 
-    if (!fGood) throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid private key");
+    if (!fGood) throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid private key encoding");
 
     CKey key = vchSecret.GetKey();
+    if (!key.IsValid()) throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Private key outside allowed range");
+
     CPubKey pubkey = key.GetPubKey();
     CKeyID vchAddress = pubkey.GetID();
     {