(svn r14080) -Fix (r14052): assert triggered when drawing chat window with 32bpp-anim blitter (backup buffer was too small)

src/blitter/null.hpp

@@ -28,6 +28,7 @@ public:
 	/* virtual */ Blitter::PaletteAnimation UsePaletteAnimation() { return Blitter::PALETTE_ANIMATION_NONE; };
 
 	/* virtual */ const char *GetName() { return "null"; }
+	/* virtual */ int GetBytesPerPixel() { return 0; }
 };
 
 class FBlitter_Null: public BlitterFactory<FBlitter_Null> {