libelf: Handle elf_strptr on section without any data

libelf: Handle elf_strptr on section without any data

In the unlikely situation that elf_strptr was called on a section with
sh_size already set, but that doesn't have any data yet we could crash
trying to verify the string to return.

This could happen for example when a new section was created with
elf_newscn, but no data having been added yet.

	* libelf/elf_strptr.c (elf_strptr): Check strscn->rawdata_base
	is not NULL.

https://sourceware.org/bugzilla/show_bug.cgi?id=32672

Signed-off-by: Mark Wielaard <mark@klomp.org>