Avoid failing on shutdown executor

[yschimke]

Testing of https://github.com/square/okhttp/issues/7410

Consider this fix for discussion. Open to suggestions.

Issue happens because RealCall.executeOn has rejected execute call and falls back to recursive handling.

[yschimke]

Pretty sure this is not the right fix, since Executor could be shutdown at any point. So will need to change AsyncCall.executeOn.

[yschimke]

Since promoteAndExecute is in the recursive path, this should work. But still open for suggestions here.

[yschimke]

@swankjesse happy to revisit after your review. In the mean time, give it some time to bake. Should only have any effect, after Executor is shutdown, so not likely to cause problems in normal use.