Fix dstr_own() when the dstr already holds a string.

src/dstr.c

@@ -176,7 +176,7 @@ void uc_dstr_own(struct DStr *str, char *restrict s)
     size_t len = strlen(s);
 
     if (str->str) {
-        free(str);
+        free(str->str);
     }
 
     str->len = len;