segtree: Fix memory leaks

segtree: Fix memory leaks

This fixes memory leaks in three places:

* set_overlap():

The allocated intervals have to be freed again before returning to
caller. While being at it, reduce indenting level in said function to
stay below 80 columns boundary.

* range_is_prefix():
* interval_map_decompose():

GMP documentation suggests to call mpz_clear() for all mpz_t type
variables once they are not used anymore to free the space they occupy.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>