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_pyramid.c

@@ -21,7 +21,7 @@
 #include "main.h"
 #include "load_save.h"
 #include "script.h"
-#include "malloc.h"
+#include "alloc.h"
 #include "overworld.h"
 #include "event_scripts.h"
 #include "constants/battle_frontier.h"
@@ -1541,7 +1541,7 @@ void sub_81AA1D8(void)
     u8 var0, var1;
 
     for (i = 0; i < 8; i++)
-        gSaveBlock2Ptr->frontier.field_CB4[i] |= 0xFFFF;
+        gSaveBlock2Ptr->frontier.field_CB4[i] |= INVALID_U16;
 
     id = sub_81AA9E4();
     sub_81AA33C(&var0, &var1);