target-cris/translate.c: fix out of bounds read

In function t_gen_mov_TN_preg and t_gen_mov_preg_TN, The begin check about the
validity of in-parameter 'r' is useless. We still access cpu_PR[r] in the
follow code if it is invalid. Which will be an out-of-bounds read error.

Fix it by using assert() to ensure it is valid before using it.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
