Fix T95332: Crash loading older files.

Image buffer was visible but buffer wasn't available. In the case
the color only overlay of the render result was displayed the image
buffer was not check to be valid.

This patch adds a null pointer check to check in `IMB_alpha_affects_rgb`
to solve this crash.

source/blender/imbuf/intern/divers.c

@@ -104,7 +104,7 @@ MINLINE void float_to_byte_dither_v4(
 
 bool IMB_alpha_affects_rgb(const ImBuf *ibuf)
 {
-  return (ibuf->flags & IB_alphamode_channel_packed) == 0;
+  return ibuf && (ibuf->flags & IB_alphamode_channel_packed) == 0;
 }
 
 void IMB_buffer_byte_from_float(uchar *rect_to,