Add assertions to in IPC::Connection to make sure we have an error if decoder is null, since we've had a recent bug in this area (265736@main).
Pull request: https://github.com/WebKit/WebKit/pull/15654
Committed 265925@main (19cf6854df35): <https://commits.webkit.org/265925@main> Reviewed commits have been landed. Closing PR #15654 and removing active labels.
<rdar://problem/112045942>