[RuntimeDyld] Don't try to get the contents of sections that don't have any

(e.g. bss sections).

MachO and ELF have been silently letting this pass, but COFFObjectFile contains
an assertion to catch this kind of (ab)use of the getSectionContents, and this
was causing the JIT to crash on COFF objects with BSS sections. This patch
should fix that.

llvm-svn: 250371