checkpolicy: check before potential NULL dereference

policy_define.c: In function ‘define_te_avtab_extended_perms’:
    policy_define.c:1946:17: error: potential null pointer dereference [-Werror=null-dereference]
     1946 |         r->omit = omit;
          |                 ^

In the case of `r` being NULL, avrule_read_ioctls() would return
with its parameter `rangehead` being a pointer to NULL, which is
considered a failure in its caller `avrule_ioctl_ranges`.
So it is not necessary to alter the return value.

Found by GCC 11 with LTO enabled.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>

checkpolicy/policy_define.c

@@ -1943,7 +1943,9 @@ int avrule_read_ioctls(struct av_ioctl_range_list **rangehead)
 		}
 	}
 	r = *rangehead;
-	r->omit = omit;
+	if (r) {
+		r->omit = omit;
+	}
 	return 0;
 error:
 	yyerror("out of memory");