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/rotating_gate.c

@@ -461,7 +461,7 @@ static const union AffineAnimCmd *const sSpriteAffineAnimTable_RotatingGate[] =
 static const struct SpriteTemplate sSpriteTemplate_RotatingGateLarge =
 {
     .tileTag = ROTATING_GATE_TILE_TAG,
-    .paletteTag = 0xFFFF,
+    .paletteTag = INVALID_U16,
     .oam = &sOamData_RotatingGateLarge,
     .anims = sSpriteAnimTable_RotatingGateLarge,
     .images = NULL,
@@ -472,7 +472,7 @@ static const struct SpriteTemplate sSpriteTemplate_RotatingGateLarge =
 static const struct SpriteTemplate sSpriteTemplate_RotatingGateRegular =
 {
     .tileTag = ROTATING_GATE_TILE_TAG,
-    .paletteTag = 0xFFFF,
+    .paletteTag = INVALID_U16,
     .oam = &sOamData_RotatingGateRegular,
     .anims = sSpriteAnimTable_RotatingGateRegular,
     .images = NULL,