bundle-uri: plug leak in unbundle_from_file()

bundle-uri: plug leak in unbundle_from_file()

The function `unbundle_from_file()` has two memory leaks:

  - We do not release the `struct bundle_header header` when hitting
    errors because we return early without any cleanup.

  - We do not release the `struct strbuf bundle_ref` at all.

Plug these leaks by creating a common exit path where both of these
variables are released.

While at it, refactor the code such that the variable assignments do not
happen inside the conditional statement itself according to our coding
style.

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