CRIS: fix PR ld/26589, a missing NULL check in fix for PR ld/22269

Not sure why there wasn't a NULL check in the ld/22269 patch
(e01c16a8) at the time, as there was one for the corresponding patch
to elf32-m68k.c (5056ba1d).

Incidentally, I had missed that in 2017, as a prerequisite for the
ld/22269 series, the check_relocs function finally were made "safe"!
(I.e. the number of references and symbol types are final, garbage
collection done, so port-specific accounting can be made sanely.)

Committed.

bfd:
	PR ld/26589
	* elf32-cris.c (cris_elf_check_relocs): Add missing NULL check
	on argument before calling UNDEFWEAK_NO_DYNAMIC_RELOC.

ld:
	PR ld/26589
	* testsuite/ld-elf/pr26589.d, testsuite/ld-elf/locref3.s: New test.