tar: fix exitcode check for MMU-spawned unpacking helpers

Testcase: tar xvzf EMPTY_FILE

function                                             old     new   delta
open_transformer                                     102     106      +4
get_header_tar                                      1781    1785      +4

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