segtree: wrong prefix expression length on interval_map_decompose()

segtree: wrong prefix expression length on interval_map_decompose()

interval_map_decompose() sets expr->len to zero. This causes problems
from expr_to_intervals() that calls range_expr_value_high() and
calculates:

	 expr->len - expr->prefix_len

this operation underflows, then mpz_init_bitmask() allocates a huge
bitmask.

Use expr_value(i)->len given that we already use this to calculate the
prefix length.

Reported-by: Richard Mörbitz <richard.moerbitz@tu-dresden.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>