emacs-27 366a97c: Avoid crashes when trying to load bad GIF files

[Eli Zaretskii]

branch: emacs-27
commit 366a97c980f068a3ed665d337d644ee58cd25ca0
Author: Eli Zaretskii <eliz@gnu.org>
Commit: Eli Zaretskii <eliz@gnu.org>

    Avoid crashes when trying to load bad GIF files
    
    * src/image.c (gif_load): Handle the case when GifErrorString
    returns NULL.  (Bug#43281)
---
 src/image.c | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/src/image.c b/src/image.c
index 56878bc..956fb13 100644
--- a/src/image.c
+++ b/src/image.c
@@ -8160,11 +8160,13 @@ gif_load (struct frame *f, struct image *img)
       if (gif == NULL)
        {
 #if HAVE_GIFERRORSTRING
-         image_error ("Cannot open `%s': %s",
-                      file, build_string (GifErrorString (gif_err)));
-#else
-         image_error ("Cannot open `%s'", file);
+         const char *errstr = GifErrorString (gif_err);
+         if (errstr)
+           image_error ("Cannot open `%s': %s", file, build_string (errstr));
+         else
 #endif
+         image_error ("Cannot open `%s'", file);
+
          return 0;
        }
     }
@@ -8190,11 +8192,13 @@ gif_load (struct frame *f, struct image *img)
       if (!gif)
        {
 #if HAVE_GIFERRORSTRING
-         image_error ("Cannot open memory source `%s': %s",
-                      img->spec, build_string (GifErrorString (gif_err)));
-#else
-         image_error ("Cannot open memory source `%s'", img->spec);
+         const char *errstr = GifErrorString (gif_err);
+         if (errstr)
+           image_error ("Cannot open memory source `%s': %s",
+                        img->spec, build_string (errstr));
+         else
 #endif
+         image_error ("Cannot open memory source `%s'", img->spec);
          return 0;
        }
     }
@@ -8474,9 +8478,9 @@ gif_load (struct frame *f, struct image *img)
       if (error_text)
        image_error ("Error closing `%s': %s",
                     img->spec, build_string (error_text));
-#else
-      image_error ("Error closing `%s'", img->spec);
+      else
 #endif
+      image_error ("Error closing `%s'", img->spec);
     }
 
   /* Maybe fill in the background field while we have ximg handy. */