object-file-convert: don't leak when converting tag objects

object-file-convert: don't leak when converting tag objects

Upon close examination I discovered that while brian's code to convert
tag objects was functionally correct, it leaked memory.

Rearrange the code so that all error checking happens before any
memory is allocated.

Add code to release the temporary strbufs the code uses.

The code pretty much assumes the tag object ends with a newline,
so add an explict test to verify that is the case.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>