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

disas/cris.c

@@ -2490,7 +2490,7 @@ print_with_operands (const struct cris_opcode *opcodep,
 	const struct cris_spec_reg *sregp
 	  = spec_reg_info ((insn >> 12) & 15, disdata->distype);
 
-	if (sregp->name == NULL)
+	if (sregp == NULL || sregp->name == NULL)
 	  /* Should have been caught as a non-match earlier.  */
 	  *tp++ = '?';
 	else