Verify sizes when trying to read a BitcodeAbbrevOp

Summary:
Make sure the abbrev operands are valid and that we can read/skip them
afterwards.

Bug found with AFL fuzz.

Reviewers: rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9030

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235595 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed