Avoid crashing on invalid or unreadable ELF core

Issue discovered during the GSoC 2014 project implementing FreeBSD
kernel support.  The existing elf-core Process plugin crashed trying
to read from /dev/mem (the kernel memory device).

Patch by Mike Ma.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@211102 91177308-0d34-0410-b5e6-96231b3b80d8