encoding/gob: prevent panic from index out of range in Decoder.typeString

encoding/gob: prevent panic from index out of range in Decoder.typeString

I believe this bug is introduced by CL 460543 which optimizes the allocations
by changing the type of `idToType` from map to slice, but didn't update the
access code in `Decoder.typeString` that is safe for map but not for slice.

Fixes #62117

Change-Id: I0f2e4cc2f34c54dada1f83458ba512a6fde6dcbe
Reviewed-on: https://go-review.googlesource.com/c/go/+/520757
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>