Fix small file passthrough

[cgbur]

Fixes passthrough on files with fewer than 4 magic bytes by checking if passthrough is enabled before exiting with an error. If force is enabled and destination is stdout, the bytes are copied as-is to the output.

Changes:

• add a check for passthrough if we read fewer than 4 bytes (zstd magic length)
• add a test to ensure passthrough works on short files

fixes: #3210
review @terrelln

[terrelln]

Awesome! Thanks for the PR!