Readobj: If NumbersOfSections is 0xffff, it's an COFF import library.

0xffff does not mean that there are 65535 sections in a COFF file but
indicates that it's a COFF import library. This patch fixes SEGV error
when an import library file is passed to llvm-readobj.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194844 91177308-0d34-0410-b5e6-96231b3b80d8