MFC r295749:

Reorganize the handling all-zeroes terminal block in sparse mode

PR: 189284
(original PR whose fix introduced this bug)

PR: 207092

MFC rev. 265593:
(dd_out): Fix handling of all-zeroes block at end of input with
conv=sparse.

PR: bin/189174
PR: bin/189284
Reviewed by: kib

MFC rev. 265593:
(dd_out): Fix handling of all-zeroes block at end of input with
conv=sparse.

PR: bin/189174
PR: bin/189284
Reviewed by: kib