BLI_path: assert passing relative paths to BLI_path_cmp_normalized

Assert when "//" prefixed relative paths are passed to
BLI_path_cmp_normalized as this can't be expanded
and it's possible the paths come from different blend files.

source/blender/blenlib/intern/path_util.c

@@ -1832,6 +1832,8 @@ void BLI_path_slash_native(char *path)
 
 int BLI_path_cmp_normalized(const char *p1, const char *p2)
 {
+  BLI_assert_msg(!BLI_path_is_rel(p1) && !BLI_path_is_rel(p2), "Paths arguments must be absolute");
+
   /* Normalize the paths so we can compare them. */
   char norm_p1[FILE_MAX];
   char norm_p2[FILE_MAX];