reduce moves consumed by speed addiction to maximal 20

The addiction effects for speed-addiction consumes up to 30 moves from `u.moves` each turn (and very rarely more) with addiction intensity of 6 and above. But minimal speed for the player is 25 moves per turn (player.cpp:1232).

This means an effective loose of 5 moves per turn, which makes `u.moves` goes into negatives until the addition is over and makes it impossible for the player to do anything.

fixes #5441