RISC-V: vsetvl: Add null check for fault-first loop [PR122652].

RISC-V: vsetvl: Add null check for fault-first loop [PR122652].

For a fault-first load we store the first instruction that read its VL
result.  The loop to do so uses next_nondebug_insn () which returns
nullptr when we are at the end.  Check for that before accessing the
next insn.

	PR target/122652

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc: Add nullptr check.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/pr122652.c: New test.