debug/gocore: defend againt partial DWARF records

Make sure we don't crash if AttrField returns nil.

I needed a few of these fixed to read 1.10 core dumps.

Change-Id: I410cdf4a9a524c12c4f77c75bbeba35ca007bcb6
Reviewed-on: https://go-review.googlesource.com/100056
Reviewed-by: Heschi Kreinick <heschi@google.com>