strbuf: fix leak when `appendwholeline()` fails with EOF

strbuf: fix leak when `appendwholeline()` fails with EOF

In `strbuf_appendwholeline()` we call `strbuf_getwholeline()` with a
temporary buffer. In case the call returns an error we indicate this by
returning EOF, but never release the temporary buffer. This can cause a
leak though because `strbuf_getwholeline()` calls getline(3). Quoting
its documentation:

    If *lineptr was set to NULL before the call, then the buffer
    should be freed by the user program even on failure.

Consequently, the temporary buffer may hold allocated memory even when
the call to `strbuf_getwholeline()` fails.

Fix this by releasing the temporary buffer on error.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>