accel/tcg: Test CPUJumpCache in tb_jmp_cache_clear_page

After commit 4e4fa6c1 ("accel/tcg: Complete cpu initialization
before registration"), it looks the CPUJumpCache pointer can be NULL.
This causes a SIGSEV when running debug-wp-migration kvm unit test.

At the first place it should be clarified why this TCG code is called
with KVM acceleration. This may hide another bug.

Fixes: 4e4fa6c1 ("accel/tcg: Complete cpu initialization before registration")
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Message-Id: <20230203171510.2867451-1-eric.auger@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>