debug/dwarf: better error detection in parseUnits

debug/dwarf: better error detection in parseUnits

Tweak the (*Data).parseUnits method to check a bit more carefully for
buffer read errors, so as to avoid infinite looping on malformed
inputs.

Fixes #51758.
Updates #47653.

Change-Id: I6d67fcb53392acf651ceec636789ab9e49ad5a5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/393874
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>