In some cases the pointer returned is used directly without further
error checking.
As most instances already assert() in this case this commit simply
adds asserts where missing and deemed appropriate.
Rename "player" directory to "players", and reintroduce the "player_"
file prefix. This has been done after sev's request.
"players/player_foo.*" is more descriptive, and avoids potential name
clashes in libraries
Adrian Frühwirth
Colin Snover
athrxx
Johannes Schickel
Filippos Karapetis