driver_settings: fix allocating an empty settings

I misread the condition and broke this in 0687a01. Thanks to Axel for
reviewing!
* Refactor the code again to move all the error checking at the top of
the function, to make it easier to read.

src/system/libroot/os/driver_settings.cpp

@@ -828,21 +828,20 @@ load_driver_settings_file(int fd)
 void *
 parse_driver_settings_string(const char *settingsString)
 {
-	if (settingsString == NULL)
+	char *text = NULL;
-		return NULL;
+	if (settingsString != NULL) {
-
 		// we simply copy the whole string to use it as our internal buffer
-	char *text = strdup(settingsString);
+		text = strdup(settingsString);
-	if (text != NULL) {
+		if (text == NULL)
+			return NULL;
+	}
+
 	settings_handle *handle = new_settings(text, NULL);
 	if (handle == NULL)
 		free(text);
 	return handle;
 }
 
-	return NULL;
-}
-
 
 /*!
 	This function prints out a driver settings structure to a human