src/liboping.c: Fixed argument verification.
src/liboping.c: Fixed argument verification.

ping_iterator_get_info()'s 'buffer' argument _may_ be NULL, since the user
might only be interested in the size of the (to be) returned value. If
'buffer' is NULL, '*buffer_len' should be 0, though, to make sure nothing gets
written to 'buffer'.

This bug was introduced in commit 9ae85a09d919777c9be52e55b23c08bf2c8c1d0e.

iseki-masaya/liboping,iseki-masaya/liboping,barak/liboping,octo/liboping,barak/liboping,iseki-masaya/liboping,octo/liboping,octo/liboping,barak/liboping,octo/liboping,iseki-masaya/liboping