The man page says, it's not optional argument:
  The application shall ensure that the infop argument
  points to a siginfo_t structure.

Fixes:
Test output: ================================
17:40:26.128:     5: FAIL: helper_zombie_child.c:33: waitid (errno = 14 (Bad address))
17:40:26.128:     4: FAIL: helper_zombie_child.c:78: read (errno = 2 (No such file or directory))
17:40:26.129:     3: ERR: test.c:229: Test exited unexpectedly with code 1

Test output: ================================
15:30:49.021:    30: ERR: sigpending.c:213: waitid (errno = 14 (Bad address))
15:30:49.021:    29: ERR: test.c:229: Test exited unexpectedly with code 1

and etc.

travis-ci: success for 32-bit tests fixes
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>