Fix out of bounds memory read in get_keymap_from_resource.

The function reads resource content into a buffer whose size matches the
size of the file contents. This buffer does not have an extra byte that
would 0 terminate this string. This is by itself is not a problem.
Unfortunately the buffer is passed to g_utf8_make_valid function with
size argument specified as -1 which means the buffer is supposed to be
NULL terminated. The end result is g_utf8_make_valid will read at least
1 byte past "contents" buffer size.

Fix this by specifying buffer size when calling g_utf8_make_valid.

eekboard/eekboard-context-service.c

@@ -124,7 +124,7 @@ get_keymap_from_resource(const gchar *keyboard_type, gboolean fallback)
                                   &bytes_read, NULL, &error))
         goto keymap_error;
 
-    return g_utf8_make_valid (contents, -1);
+    return g_utf8_make_valid (contents, size);
 
 keymap_error:
     if (fallback)