Stream selection doesn't check the existence of the stream

[burek]

Summary of the bug:

When the stream selection is used in such a way that it selects a non-existing stream, ffmpeg doesn't show any warnings or errors, but rather chooses to ignore the issue completely.

For example, if our input contains 2 video streams and we try to select a non-existent video stream like "-c:v:333" it will not complain, but IMHO it should, because if a user has specified a command line option for something, it means he wants that option to be taken into account. The least ffmpeg could do is issue a warning saying it is a non-existent stream selected.

How to reproduce:

% ffmpeg -i input -c:v:333 copy output

Instead of getting an error, warning or something like this:

Stream mapping:
  Stream #0:0 -> #0:0 (copy)

the result is the same as if "-c:v:333 copy" was deleted from cmd line, thus ignored totally, using some default encoder like this:

Stream mapping:
  Stream #0:0 -> #0:0 (h264 -> libx264)