Only consume an AVPacket when all the samples have been read.

When the rate of samples output is limited (by the default value
of max_samples), consuming the first packet immediately will cause
timing problems:

- The first packet with PTS 0 will output 4608 samples and be
consumed entirely
- The second packet with PTS 64 will output the remaining samples
(typically, a lot, that's why max_samples exist) until the decoded
samples of the first packet have been exhausted, at which point the
samples of the second packet will be decoded and output when
av_decode_frame is called with the next packet).

That means there's a PTS jump since the first packet is 'decoded'
immediately, which can be seen with avplay or mplayer: the timing
jumps immediately to 6.2s (which is the size of a packet).

Sample: http://streams.videolan.org/issues/6348/Goldwave-MAClib.ape


Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>