[gimp] app: fix a crash in PDB "gimp-image-get-colormap".

[Jehan <jehanp src gnome org>]

commit f90efdf9d6dcf950aae279aa8bbd551f6c04d214
Author: Jehan <jehan girinstud io>
Date:   Sat Mar 12 18:05:07 2022 +0100

    app: fix a crash in PDB "gimp-image-get-colormap".
    
    In case we have a palette on an image which is empty, don't return a
    colormap together with a size 0, just return NULL.

 app/core/gimpimage-colormap.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)
---
diff --git a/app/core/gimpimage-colormap.c b/app/core/gimpimage-colormap.c
index b880b6b552..7843a1235d 100644
--- a/app/core/gimpimage-colormap.c
+++ b/app/core/gimpimage-colormap.c
@@ -235,7 +235,7 @@ guchar *
 gimp_image_get_colormap (GimpImage *image)
 {
   GimpImagePrivate *private;
-  guchar           *colormap;
+  guchar           *colormap = NULL;
   gint              n_colors, i;
 
   g_return_val_if_fail (GIMP_IS_IMAGE (image), NULL);
@@ -245,15 +245,19 @@ gimp_image_get_colormap (GimpImage *image)
     return NULL;
 
   n_colors = gimp_palette_get_n_colors (private->palette);
-  colormap = g_new0 (guchar, GIMP_IMAGE_COLORMAP_SIZE);
 
-  for (i = 0; i < n_colors; i++)
+  if (n_colors > 0)
     {
-      GimpPaletteEntry *entry = gimp_palette_get_entry (private->palette, i);
-      gimp_rgb_get_uchar (&entry->color,
-                          &colormap[i * 3],
-                          &colormap[i * 3 + 1],
-                          &colormap[i * 3 + 2]);
+      colormap = g_new0 (guchar, GIMP_IMAGE_COLORMAP_SIZE);
+
+      for (i = 0; i < n_colors; i++)
+        {
+          GimpPaletteEntry *entry = gimp_palette_get_entry (private->palette, i);
+          gimp_rgb_get_uchar (&entry->color,
+                              &colormap[i * 3],
+                              &colormap[i * 3 + 1],
+                              &colormap[i * 3 + 2]);
+        }
     }
 
   return colormap;