Bug fix: signal an error if `elf_rawdata()` is used with a section of type
`SHT_NULL`.

Submitted by:	mark-jn
Ticket:		#414

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2921 95820547-d848-0410-985e-9ae8fe0fa350