debug/elf: do not read unrelated bytes for SHT_NOBITS sections

debug/elf: do not read unrelated bytes for SHT_NOBITS sections

SHT_NOBITS sections do not occupy space in the file and their offset is
"conceptual", reading their data should return all zeroes instead of
reading bytes from the section that follows them.

Change-Id: Iaa9634792c1909c3e87dab841dd646cd6dcf9027
Reviewed-on: https://go-review.googlesource.com/c/go/+/375216
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>