PECOFF: Fix section name computation
If a section name is exactly 8 bytes long (or has been truncated to 8
bytes), it will not contain the terminating nul character. This means
reading the name as a c string will pick up random data following the
name field (which happens to be the section vm size).
This fixes the name computation to avoid out-of-bounds access and adds a
Reviewers: zturner, stella.stamenova
Differential Revision: https://reviews.llvm.org/D56124