libelf: elf_strptr should fetch the shdr for the section if not yet known.

libelf: elf_strptr should fetch the shdr for the section if not yet known.

elf_strptr might be called before the shdrs are read in. In that case it
needs to explicitly call __elf[32|64]_getshdr_rdlock to check the section
type and size. The new strptr testcase triggers this corner case and crashes
before the fix.

Signed-off-by: Mark Wielaard <mjw@redhat.com>