Reuse the strict mechanism from FileLine for FuncForPC, so we don't
crash when asking the pcln table about bad pcs.

Fixes #29735

Change-Id: Iaffb32498b8586ecf4eae03823e8aecef841aa68
Reviewed-on: https://go-review.googlesource.com/c/157799Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>