Before this patch, the implementation of pipe: inputs/outputs would
silently fall back to stdin/stdout for any argument not successfully
parsed by strtol().

This patch introduces an explicit error for any non-numerical arguments,
which should avoid user confusion as in #10977.

New behavior:

$ cat /tmp/video.mkv | ./ffmpeg -i pipe:aa -acodec copy -vcodec copy -f matroska pipe:1 | cat >/tmp/out.mkv

[pipe @ 0x5618c7bcf740] Non-numerical argument "aa" to pipe:
[in#0 @ 0x5618c7bced00] Error opening input: Invalid argument
Error opening input file pipe:aa.
Error opening input files: Invalid argument

Based on the patch of Nils Goroll <nils.goroll@uplex.de>.
Signed-off-by: Marton Balint <cus@passwd.hu>