cmd/ld: fix 'use after free' error in new obj code
cmd/ld: fix 'use after free' error in new obj code

Many thanks to Elias Naur for finding this with Valgrind on Linux.
Perhaps this is what is breaking the windows/amd64 builder.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7595044

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