libthread_db: unsign map_len and use reallocarray(3).

libthread_db: unsign map_len and use reallocarray(3).

Lengths are not negative, so map_len should be unsigned. Unsign the
corresponding indexes too and bring a small use of reallocarray(3).

Reorder the memset to be consistent with the realloc: it appears we
were only clearing half the memory in pt_map_thread().

MFC after:	2 weeks