[gimp/gimp-2-10] app: fix gimp_chunk_iterator_set_priority_rect()

[Ell <ell src gnome org>]

commit 4cbec7608ff0d77d80af69c889eb99247cc5ee44
Author: Ell <ell_se yahoo com>
Date:   Sat Jan 12 07:30:43 2019 -0500

    app: fix gimp_chunk_iterator_set_priority_rect()
    
    (cherry picked from commit 942e1aa9390b163b0b4a2998a582bb0b850710e3)

 app/core/gimpchunkiterator.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/app/core/gimpchunkiterator.c b/app/core/gimpchunkiterator.c
index 5e3243b2bb..fc627bb467 100644
--- a/app/core/gimpchunkiterator.c
+++ b/app/core/gimpchunkiterator.c
@@ -270,7 +270,8 @@ gimp_chunk_iterator_set_priority_rect (GimpChunkIterator   *iter,
     {
       iter->priority_rect = *rect;
 
-      gimp_chunk_iterator_merge (iter);
+      if (gimp_chunk_iterator_prepare (iter))
+        gimp_chunk_iterator_merge (iter);
     }
 }