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_controller_wally.c

@@ -348,7 +348,7 @@ static void CompleteOnHealthbarDone(void)
 
     SetHealthboxSpriteVisible(gHealthboxSpriteIds[gActiveBattler]);
 
-    if (hpValue != -1)
+    if (hpValue != INVALID_S16)
     {
         UpdateHpTextInHealthbox(gHealthboxSpriteIds[gActiveBattler], hpValue, HP_CURRENT);
     }