Revert previous change after discussion with Campbell

Let's just be really verbose and it some compiler gives
issues tell him NOT to do this.

We shouldn't really workaround compiles' idiocy..

source/blender/blenlib/intern/boxpack2d.c

@@ -87,7 +87,7 @@ typedef struct BoxVert {
 
 BLI_INLINE int quad_flag(unsigned int q)
 {
-	BLI_assert(q < 4);
+	BLI_assert(q < 4 && q >= 0);
 	return (1 << q);
 }