perform one more check on song formats

it detects something like this: %. (where . doesn't stand for a valid tag type).

src/helpers.cpp

@@ -147,13 +147,15 @@ void ParseArgv(int argc, char **argv)
 			{
 				if (argc > ++i)
 				{
-					// apply additional pair of braces
-					now_playing_format = "{";
-					now_playing_format += argv[i];
-					now_playing_format += "}";
-					Replace(now_playing_format, "\\n", "\n");
-					Replace(now_playing_format, "\\t", "\t");
-					MPD::Song::ValidateFormat("now-playing format", now_playing_format);
+					if (MPD::Song::isFormatOk("now-playing format", now_playing_format))
+					{
+						// apply additional pair of braces
+						now_playing_format = "{";
+						now_playing_format += argv[i];
+						now_playing_format += "}";
+						Replace(now_playing_format, "\\n", "\n");
+						Replace(now_playing_format, "\\t", "\t");
+					}
 				}
 				std::cout << utf_to_locale_cpy(Mpd.GetCurrentSong().toString(now_playing_format)) << "\n";
 			}