Clean up getbalance RPC parameter handling

Only change in behavior is that unsupported combinations of parameters now
trigger more specific error messages instead of the vague "JSON value is not a
string as expected" error.

src/wallet/rpcwallet.cpp

@@ -768,18 +768,31 @@ UniValue getbalance(const JSONRPCRequest& request)
 
     LOCK2(cs_main, pwallet->cs_wallet);
 
-    if (request.params[0].isNull() && request.params[1].isNull() && request.params[2].isNull())
+    const UniValue& account_value = request.params[0];
+    const UniValue& minconf = request.params[1];
+    const UniValue& include_watchonly = request.params[2];
+
+    if (account_value.isNull()) {
+        if (!minconf.isNull()) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER,
+                "getbalance minconf option is only currently supported if an account is specified");
+        }
+        if (!include_watchonly.isNull()) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER,
+                "getbalance include_watchonly option is only currently supported if an account is specified");
+        }
         return ValueFromAmount(pwallet->GetBalance());
+    }
 
-    const std::string& account_param = request.params[0].get_str();
+    const std::string& account_param = account_value.get_str();
     const std::string* account = account_param != "*" ? &account_param : nullptr;
 
     int nMinDepth = 1;
-    if (!request.params[1].isNull())
-        nMinDepth = request.params[1].get_int();
+    if (!minconf.isNull())
+        nMinDepth = minconf.get_int();
     isminefilter filter = ISMINE_SPENDABLE;
-    if(!request.params[2].isNull())
-        if(request.params[2].get_bool())
+    if(!include_watchonly.isNull())
+        if(include_watchonly.get_bool())
             filter = filter | ISMINE_WATCH_ONLY;
 
     return ValueFromAmount(pwallet->GetLegacyBalance(filter, nMinDepth, account));