Fix some undeclared global variables in falling.lua

Fixes this:
```
08:52:31: ERROR[ServerThread]: Assignment to undeclared global "prop" inside a function at ...inetest/minetest-servers/bin/../builtin/game/falling.lua:36.
16:06:10: ERROR[ServerThread]: Assignment to undeclared global "obj" inside a function at ...inetest/minetest-servers/bin/../builtin/game/falling.lua:115. 
08:52:48: ERROR[ServerThread]: Assignment to undeclared global "n" inside a function at ...inetest/minetest-servers/bin/../builtin/game/falling.lua:166.
13:37:44: ERROR[ServerThread]: Assignment to undeclared global "p_bottom" inside a function at ...inetest/minetest-servers/bin/../builtin/game/falling.lua:168.
13:37:44: ERROR[ServerThread]: Assignment to undeclared global "n_bottom" inside a function at ...inetest/minetest-servers/bin/../builtin/game/falling.lua:169.
```