pkcs12: fix ToPEM to not ignore non-nil error from getSafeContents Fixes golang/go#19542 Change-Id: I60d2370d1d1e0763c72c9cc203ea2ff21123af73 Reviewed-on: https://go-review.googlesource.com/38261 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>