Partial fix for bug 2368 (version 3)

[Dave Higton]

This gives a more user-friendly error message (sadly I can't offer any 
translations).

Dave

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diff --git a/resources/FatMessages b/resources/FatMessages
index d8fa5f0..bed740e 100644
--- a/resources/FatMessages
+++ b/resources/FatMessages
@@ -3222,6 +3222,8 @@ fr.all.DirectoryError:répertoire '%s' existe déjà
 it.all.DirectoryError:La directory '%s' è già esistente
 nl.all.DirectoryError:map '%s' bestaat reeds
 
+en.ro.SprIsNull:Unable to convert image to sprite
+
 # Error messages for Amiga version only
 en.ami.CompError:Unable to open
 de.ami.CompError:Nicht zu öffnen
diff --git a/riscos/bitmap.c b/riscos/bitmap.c
index 384895d..162a8c6 100644
--- a/riscos/bitmap.c
+++ b/riscos/bitmap.c
@@ -43,6 +43,7 @@
 #include "utils/filename.h"
 #include "utils/log.h"
 #include "utils/utils.h"
+#include "utils/messages.h"
 #include "desktop/plotters.h"
 #include "content/content.h"
 #include "image/bitmap.h"
@@ -272,6 +273,11 @@ bool riscos_bitmap_save(void *vbitmap, const char *path, 
unsigned flags)
        struct bitmap *bitmap = (struct bitmap *) vbitmap;
        os_error *error;
 
+       if (bitmap == NULL) {
+               warn_user("SaveError", messages_get("SprIsNull"));
+               return false;
+       }
+
        if (!bitmap->sprite_area) {
                riscos_bitmap_get_buffer(bitmap);
        }