llvm-objdump: don't assert if ELF file has no sections

FreeBSD core files, for example, have no sections (only program headers).

llvm.org/pr20139
Differential Revision: http://reviews.llvm.org/D4323


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211904 91177308-0d34-0410-b5e6-96231b3b80d8