[dwarfdump] Fix off-by-one bug in accelerator table extractor.

This fixes a bug where the verifier was complaining about empty
accelerator tables. When the table is empty, its size is not a valid
offset as it points after the end of the section.

This patch also makes the extractor return llvm:Error instead of bool
for better error reporting in the verifier.

Differential revision: https://reviews.llvm.org/D41063

rdar://35932007

llvm-svn: 320399