tac: don't misbehave with multiple non-seekable inputs

tac: don't misbehave with multiple non-seekable inputs

* src/tac.c (copy_to_temp): Do not reuse the template buffer.
Instead, scribble only on a freshly-xstrdup'd copy each time.
Free that buffer both here, upon failure, and ...
(tac_nonseekable): ...free the buffer in caller, upon success.
* tests/misc/tac-2-nonseekable: New file.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Bug fixes): Mention it.
Reported by Ambrose Feinstein in http://debbugs.gnu.org/9762.