S/390: Fix segfault when linking -shared -nostartfiles

Regression-tested on s390x.

bfd/ChangeLog:

2017-07-28  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	Backport from mainline
	2017-07-28  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* elf32-s390.c (elf_s390_finish_dynamic_sections): Add NULL
	pointer check for htab->elf.irelplt.
	* elf64-s390.c (elf_s390_finish_dynamic_sections): Likewise.