Corrected data pushing logic

  * NALU; use 4 bytes to describe size. Using only 2 bytes would
    overflow if a NALU was larger than 0xffff, which is common
    for reference picture when resolution is in the full-hd ranges.

  * FLVMux; push the metadata tag so it isn't mistaken for the
    beginning of a video/audio tag.

  * Removed unused function.