runtime: check for findmoduledatap returning nil

runtime: check for findmoduledatap returning nil

The findmoduledatap function will not return nil in ordinary use, but
check for nil to try to avoid crashing when we are already crashing.

Update #11783.

Change-Id: If7b1adb51efab13b4c1a37b6f3c9ad22641a0b56
Reviewed-on: https://go-review.googlesource.com/12391
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>