debug/macho: handle missing __debug_str section
debug/macho: handle missing __debug_str section

debug/elf does the same thing, use []byte{} for
any missing sections.

Fixes issue 7510

LGTM=rsc
R=golang-codereviews, iant
CC=golang-codereviews, rsc
https://codereview.appspot.com/75230043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang