theme_menu: Error out gracefully when a directory doesn't exist

Normally we can't browse into a directory that doesn't exist; hwoever
the theme menu lets us directly browse (eg) fonts, themes, etc.  If the
target directory is not present (due to a broken installation) we
actually crash!

So, um, check and bail gracefully.

Should fix FS#13636

Change-Id: Idb28c55cf0ce3954114d056725f9992ceaf1db20

apps/menus/theme_menu.c

@@ -29,6 +29,7 @@
 #include "settings.h"
 #include "rbpaths.h"
 #include "menu.h"
+#include "dir.h"
 #include "tree.h"
 #include "list.h"
 #include "color_picker.h"
@@ -42,6 +43,7 @@
 #include "viewport.h"
 #include "statusbar-skinned.h"
 #include "skin_engine/skin_engine.h"
+#include "splash.h"
 #include "icons.h"
 #ifdef HAVE_LCD_COLOR
 #include "filetypes.h"
@@ -258,6 +260,11 @@ int browse_folder(void *param)
         .root = info->dir,
     };
 
+    if (!dir_exists(info->dir)) {
+        splash(HZ, ID2P(LANG_PLAYLIST_DIRECTORY_ACCESS_ERROR));
+        return GO_TO_PREVIOUS;
+    }
+
     /* if we are in a special settings folder, center the current setting */
     switch(info->show_options)
     {