riscv: g_assert for NULL predicate?

[Wu, Fei]

Recent commit 0ee342256af92 switches to g_assert() for the predicate()
NULL check from returning RISCV_EXCP_ILLEGAL_INST. Qemu doesn't have
predicate() for un-allocated CSRs, then a buggy userspace application
reads CSR such as 0x4 causes qemu to exit, I don't think it's expected.

  .global _start

  .text
  _start:
      csrr t3, 0x4

Thanks,
Fei.