Prevent ObjectFileELF::GetSectionHeaderInfo() from reparsing section headers.

If we have any section headers in the collection, we already parsed them.
Therefore, don't reparse the section headers when the section_headers collection
is not empty.

See this thread for more details:
http://lists.cs.uiuc.edu/pipermail/lldb-commits/Week-of-Mon-20140707/011721.html

Change by Matthew Gardiner