go/types: fix panic in allowVersion when pos is invalid

go/types: fix panic in allowVersion when pos is invalid

CL 515656 updated go/types to use file base as key in the posVers map,
but introduced a panic when the corresponding *token.File is nil.

Check that pos is valid before performing the lookup.

Fixes #61822

Change-Id: I1ac9d48c831a470de8439a50022ba5f59b3e0bed
Reviewed-on: https://go-review.googlesource.com/c/go/+/516738
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Robert Findley <rfindley@google.com>