scanner: use list_is_first() from scanner_pop_indesc()

scanner: use list_is_first() from scanner_pop_indesc()

!list_empty() always stands true since the list is never empty
when calling scanner_pop_indesc().

Check for list_is_first() which actually tells us this is the
initial input file, hence, state->indesc is set to NULL.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>