Fix datadir handling in bitcoind

This prevents premature tries to access or create the default datadir.
This is useful when the -datadir option is specified and the default
datadir is unreachable.

src/bitcoind.cpp

@@ -92,8 +92,7 @@ static bool AppInit(int argc, char* argv[])
 
     try
     {
-        if (!fs::is_directory(GetDataDir(false)))
-        {
+        if (!CheckDataDirOption()) {
             return InitError(strprintf("Specified data directory \"%s\" does not exist.\n", gArgs.GetArg("-datadir", "")));
         }
         if (!gArgs.ReadConfigFiles(error, true)) {

src/util/system.cpp

@@ -941,7 +941,7 @@ bool ArgsManager::ReadConfigFiles(std::string& error, bool ignore_invalid_keys)
 
     // If datadir is changed in .conf file:
     ClearDatadirCache();
-    if (!fs::is_directory(GetDataDir(false))) {
+    if (!CheckDataDirOption()) {
         error = strprintf("specified data directory \"%s\" does not exist.", gArgs.GetArg("-datadir", "").c_str());
         return false;
     }