When decoding an invalid typeId the associated *decEngine was not
removed from decoderMap. If the decoder was run again on the same input
a nil *decEngine was found in the map and assumed to be initialized,
resulting in a panic.

Fixes #9649

Change-Id: I5bb51808362a21c09228c2705a658f073e5b59b3
Reviewed-on: https://go-review.googlesource.com/3509Reviewed-by: Rob Pike <r@golang.org>