disas/cris.c: Fix Coverity warning about unchecked NULL

Coverity (CID 1005689) warns that we don't check that
spec_reg_info() returned non-NULL before dereferencing.
Add the check, though as the comment notes this is
a can't-really-happen case because the earlier constraint
matching should have ruled out the "unknown reg" case.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
