Foreign code can be arbitrarily aligned,
so the function before it can have
arbitrarily much padding.
We can't call pcvalue on values in the padding.

Fixes #11653.

Change-Id: I7d57f813ae5a2409d1520fcc909af3eeef2da131
Reviewed-on: https://go-review.googlesource.com/12550Reviewed-by: Rob Pike <r@golang.org>