Seeking to invalid position doesn't report failure

[raidho36]

Seeking to invalid position doesn't report failure, and falls back to playing from 0 - making it impossible to tell if seek position was invalid. The library already detects invalid position, but it should also notify user that it wasn't valid. Many other playback libraries (not necessarily module players) fail to produce output when seeked to invalid position, so there's a suggestion that libmodplug does the same.