[gimp/metadata-browser] file-png: Add comments about the struct checks

[Roman Joost <romanofski src gnome org>]

commit 0868201f5780a168ce50ce3a5e13041b9642c7a9
Author: Mukund Sivaraman <muks banu com>
Date:   Tue Sep 27 19:22:50 2011 +0530

    file-png: Add comments about the struct checks

 plug-ins/common/file-png.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/plug-ins/common/file-png.c b/plug-ins/common/file-png.c
index 6f50eb6..ae78aca 100644
--- a/plug-ins/common/file-png.c
+++ b/plug-ins/common/file-png.c
@@ -722,6 +722,9 @@ load_image (const gchar  *filename,
   pp = png_create_read_struct (PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
   if (!pp)
     {
+      /* this could happen if the compile time and run-time libpng
+         versions do not match. */
+
       g_set_error (error, 0, 0,
                    _("Error creating PNG read struct while saving '%s'."),
                    gimp_filename_to_utf8 (filename));
@@ -1257,6 +1260,9 @@ save_image (const gchar  *filename,
   pp = png_create_write_struct (PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
   if (!pp)
     {
+      /* this could happen if the compile time and run-time libpng
+         versions do not match. */
+
       g_set_error (error, 0, 0,
                    _("Error creating PNG write struct while saving '%s'."),
                    gimp_filename_to_utf8 (filename));