Remove pmomz from players who are on ground.

Assume that every frame the player is on the ground, their pmomz will
be re-set properly if the floor is moving, therefore if the platform
STOPS, we need this to set it to 0.

src/p_user.c

@@ -9391,4 +9391,7 @@ void P_PlayerAfterThink(player_t *player)
 		player->mo->flags2 |= MF2_DONTDRAW;
 		player->mo->flags |= MF_NOGRAVITY;
 	}
+
+	if (P_IsObjectOnGround(player->mo))
+		player->mo->pmomz = 0;
 }