src: fix off-by-one error in native SetImmediate

Previously, the throwing callback would have been re-executed in case
of an exception. This patch corrects the calculation to exclude the
callback.

PR-URL: https://github.com/nodejs/node/pull/28082
Fixes: https://github.com/nodejs/node/issues/26754


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: James M Snell <jasnell@gmail.com>