canonicalise_opt must always return heap memory.

Thanks to Max Julian Hofmann for spotting this.

src/option.c

@@ -654,7 +654,7 @@ static char *canonicalise_opt(char *s)
     return 0;
 
   if (strlen(s) == 0)
-    return "";
+    return opt_string_alloc("");
 
   unhide_metas(s);
   if (!(ret = canonicalise(s, &nomem)) && nomem)