[input] Make input_resume() not reopen tracks that have finished playing

If playback was paused during the very last part of the track, the rest of the
track would be read into the input buffer and the input would be closed. With
this commit the input will not be reopened.

Also allow input_flush to be called with null argument.

src/input.h

@@ -221,7 +221,8 @@ void
 input_stop(void);
 
 /*
- * Flush input buffer. Output flags will be the same as input_read().
+ * Flush input buffer. Output flags will be the same as input_read(). Call with
+ * null pointer is valid.
  */
 void
 input_flush(short *flags);