bintrans: Error out if writing to the output failed.

[des]

Summary

• Cover all code paths.
• When decoding, check all output files, not just the last one.
• A simple ferror() check is not enough as an error may later occur while flushing whatever remains in the output buffer.

MFC after: 1 week
Sponsored by: Klara, Inc.

[des]

This needs a little more work as base64_decode() may return early.

[des]

fix decode cases correctly

[des]

reset outfile too

[allanjude]

Reviewed-by: allanjude