util: Limit size of wl_map

Since server IDs are basically indistinguishable from really big client
IDs at many points in the source, it's theoretically possible to overflow
a map and either overflow server IDs into the client ID space, or grow
client IDs into the server ID space. This would currently take a massive
amount of RAM, but the definition of massive changes yearly.

Prevent this by placing a ridiculous but arbitrary upper bound on the
number of items we can put in a map: 0xF00000, somewhere over 15 million.
This should satisfy pathological clients without restriction, but stays
well clear of the 0xFF000000 transition point between server and client
IDs. It will still take an improbable amount of RAM to hit this, and a
client could still exhaust all RAM in this way, but our goal is to prevent
overflow and undefined behaviour.

Fixes #224

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>