llvm-cov: Slightly improved error checking.

- readInt() should check all 4 bytes can be read, not just 1.
- In the event of false data in the gcno file, it was possible to index
  into a non-existent index of SmallVector, causing assertion error.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194639 91177308-0d34-0410-b5e6-96231b3b80d8