zzip: fseeko: fix comparison between incompatible pointer types

zzip: fseeko: fix comparison between incompatible pointer types

The C standard does not allow for comparing incompatible pointer types
with each other. This causes the C compiler to emit a warning when we
try to compare a pointer of type `struct zzip_disk64_trailer` (cast to
`void *`) with a pointer of type `unsigned char`.

Fix this by comparing `p + sizeof(trailer)` with `buffer + mapsize`
instead of using the cast struct.