Fix alloc.c as per #386, define INVALID_ constants and rename malloc to alloc as per #325

Some of the INVALID_ are likely erroneously placed, due to lack of documentation

src/battle_anim_utility_funcs.c

@@ -3,7 +3,7 @@
 #include "contest.h"
 #include "gpu_regs.h"
 #include "graphics.h"
-#include "malloc.h"
+#include "alloc.h"
 #include "palette.h"
 #include "sound.h"
 #include "sprite.h"
@@ -82,7 +82,7 @@ void sub_8116664(u8 taskId)
         animBattlers[1] = gBattleAnimTarget;
         break;
     case 5:
-        animBattlers[0] = 0xFF;
+        animBattlers[0] = INVALID_U8;
         break;
     case 6:
         selectedPalettes = 0;