main : fix file existence check in main.cpp (#1889)

main : fix file existence check in main.cpp (#1889)

In commit dda4b0e of PR #1872, I've introduced a check for the
existence of files before loading the model. However, I haven't
considered the case where whisper.cpp might read from stdin as well,
and in such cases, the checks should ignore the "-" argument as it
does not represent a regular file.

Additionally, this commit removes the usage of 'stat()' in favor of
the recently introduced function 'is_file_exist()' in common.cpp from
PR #1871.

Apologies for the bug introduced in the previous PR and any
inconvenience it may have caused.