raw_ostream::operator<<(StringRef): Avoid potential overflow in pointer arithmetic.

(OutBufCur + Size) might overflow if Size were large. For example on i686-linux,

  OutBufCur: 0xFFFDF27D
  OutBufEnd: 0xFFFDF370
  Size:      0x0002BF20 (180,000)

It caused flaky error in MC/COFF/section-name-encoding.s.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207621 91177308-0d34-0410-b5e6-96231b3b80d8