Resolve GIMP segfault from accessing memory past end of pixbuf

Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/2684

GIMP segfaults while switching themes between dark and gray and inputting Ctrl-O
to open a file. This is because p advances past end of pixbuf in pixbuf-render.c
compute_hint() with num_channels = 3 (no alpha). This is resolved by fixing the
if statement to only check for alpha, thereby advancing p, if there is an alpha
channel.

modules/engines/pixbuf/pixbuf-render.c

@@ -603,7 +603,7 @@ compute_hint (GdkPixbuf *pixbuf,
 	  if (r != *(p++) ||
 	      g != *(p++) ||
 	      b != *(p++) ||
-	      (n_channels != 4 && a != *(p++)))
+	      (n_channels == 4 && a != *(p++)))
 	    {
 	      hints &= ~THEME_CONSTANT_ROWS;
 	      if (!(hints & THEME_MISSING))