Object, COFF: Clean up formatting in hasExtendedRelocations

No functionality changed intended.
Object, COFF: Fix some theoretical bugs

getObject didn't consider the case where a pointer came before the start
of the object file.  No test is included, trying to come up with
something reasonable.
Object, COFF: Cleanup some code in getSectionName

Use StringRef::startswith to tidy up some code, no functionality change
intended.
Object, COFF: getRelocationSymbol shouldn't assert

lib/Object is supposed to be robust to malformed object files.  Don't
assert if we don't have a symbol table.  I'll try to come up with a test
case later.
llvm-readobj, COFF: Remove an unused variable

printRelocation doesn't use the section contents.  No functionality
change intended.