tar: fix --to-command wrt short writes

tar: fix --to-command wrt short writes

function                                             old     new   delta
bb_copyfd_exact_size                                  51      98     +47
bb_full_fd_action                                    362     394     +32
get_header_tar                                      1546    1558     +12
data_extract_to_command                              430     439      +9
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(add/remove: 0/0 grow/shrink: 4/0 up/down: 100/0)             Total: 100 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>