Fix #224: Segfault while using Ds\Set

The string may be refcounted or interned, so freeing is not valid, it
should be releasing the string. Also switch to the _func variant as I
first believed that we could use the zval_get_tmp_string() function but
that would be pointless here as IS_STRING is already handled.

Author: ndossche

src/common.c

@@ -157,9 +157,9 @@ void smart_str_appendz(smart_str *buffer, zval *value)
             return;
     }
 
-    zend_string *str = zval_get_string(value);
+    zend_string *str = zval_get_string_func(value);
     smart_str_append(buffer, str);
-    zend_string_free(str);
+    zend_string_release_ex(str, false);
 }
 
 zend_string *ds_join_zval_buffer(