Fix nullptr crash in player status hud due to wrong player item pocket access

[Brockengespenst]

Fixes a crash if multiple players are in the game and the first player does not have any bonus while other players have a bonus. Due to a wrong access of m_bonus_sprites, a nullptr is returned for the sprite pointer which causes a segfault when the hud tries to draw the sprite.

[Brockengespenst]

To be honest, I have not checked the whole item pocket logic, but I encountered a crash in local multiplayer mode. First player did not have any bonus while the second player gained a bonus and that caused the game to crash: While iterating the players, always the sprite for the first player was retrieved and drawn. As there is no sprite for BONUS_NONE, a nullptr was introduced to the map and the game crashed as it tried to draw a sprite from a nullptr.
I assume the check for if (m_bonus_sprites.find(m_player_status.m_item_pockets[i]) != m_bonus_sprites.end()) should check for the correct sprite, so I used this as well when retrieving the actual sprite. Additionally, I added a nullptr check, just to be sure.
@MatusGuy : Please check if there is anything wrong with my understanding.

[MatusGuy]

Oh my god wahahaha That slipped right through me indeed. Nice catch!