S/390: Fix testsuite segfault added with recent pgste patch.

S/390: Fix testsuite segfault added with recent pgste patch.

The recent pgste patch caused several testcases to fail with a
segfault.  Fixed with this patch by adding NULL pointer checks.

regression-tested on s390x.

bfd/ChangeLog:

2017-06-26  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* elf64-s390.c (elf_s390_additional_program_headers): Add NULL
	pointer checks.
	(elf_s390_modify_segment_map): Likewise.
	(bfd_elf_s390_set_options): Lisewise.