On iOS 11, encoding a frame may return error with log
"Error encoding frame 0", which means vtenc_output_callback
is called with status=0 and sample_buffer=NULL. Then the
encoding session will be crashed on next callback wether or not
closing the codec context.

Let us look through the link below introducing VTCompressionOutputCallback,

https://developer.apple.com/documentation/videotoolbox/vtcompressionoutputcallback?language=objc

"status=0" (noErr) means compression was successful.
"sampleBuffer=NULL" means the frame was dropped when compression
was successful (status=0) or compression was not successful (status!=0).

So we should not set AVERROR_EXTERNAL on "status=0" and "sample_buffer=NULL"
as it is not a error.

The fix is that we only set AVERROR_EXTERNAL with status value non zero.
When sample_buffer is NULL and status value is zero, we simply return
with no other operation.

This crash often occurs on iOS 11 for example encoding 720p@25fps.
Signed-off-by: sharpbai <sharpbai@gmail.com>
Signed-off-by: Rick Kern <kernrj@gmail.com>