maint: avoid new coverity warnings

maint: avoid new coverity warnings

* src/sync.c (sync_arg): Initialise variable to avoid
unitialized access if assert is disabled.
* src/head.c (elide_tail_bytes_file): Support this function
with ---presume-input-pipe and larger files,
which regressed with commit v8.23-47-g2662702.
(elide_tail_lines_file): Likewise.
* src/dd.c (dd_copy): Explicitly don't try to ftruncate()
upon failure to lseek() (the existing check against
st_size was already protecting that).
* src/factor.c (factor_using_squfof): Assert (only when
linting due to performance) to avoid the implication of
divide by zero.
* src/od.c (read_block): Remove dead code.
* src/tac.c (tac_seekable): Likewise.
* src/ls.c (gobble_file): Likewise.