PVS-Studio warning V595 for PHP

static int user_shutdown_function_call(zval *zv TSRMLS_DC)
{
  ....
  php_error(E_WARNING, "....", function_name->val);  //<==
  if (function_name) {                               //<==
    STR_RELEASE(function_name);
  }
  ....
}

This suspicious code was found in PHP project by PVS-Studio static code analyzer.
Warning message is:
V595 The 'function_name' pointer was utilized before it was verified against nullptr. Check lines: 4859, 4860. basic_functions.c 4859

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