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

include/global.h

@@ -25,6 +25,14 @@
 #define INCBIN_S32 {0}
 #endif // IDE support
 
+// Invalid / Out of Bound Placeholder values
+#define INVALID_U8 0xFF
+#define INVALID_U16 0xFFFF
+#define INVALID_U32 0xFFFFFFFF
+#define INVALID_S8 -1
+#define INVALID_S16 -1
+#define INVALID_S32 -1
+
 #define ARRAY_COUNT(array) (size_t)(sizeof(array) / sizeof((array)[0]))
 
 #define SWAP(a, b, temp)    \