Do not parse DIE's outside a compilation units range.

In DWARFCompileUnit::ExtractDIEsIfNeeded we are relying on a compilation units
DIEs to be terminated by a null entry.  I think the standard is fairly clear
that all sibling chains are to be terminated by null, but at least gcc 4.5.2
disagrees -- the top level chain drops the final entry.  This results in us
interpreting the next compilation unit header as a DIE.

Regardless of whether gcc is right or wrong, we should not overstep a
compilation units extent.  This patch ensures that we do not attempt to extract
a DIE beyond the length specified for a given DWARFCompileUnit by ensuring our
current offset is strictly less than the start of the next CU.

llvm-svn: 131721