target/i386: Verify memory operand for lcall and ljmp

These two opcodes only allow a memory operand.

Lacking the check for a register operand, we used the A0 temp
without initialization, which led to a tcg abort.

Buglink: https://bugs.launchpad.net/qemu/+bug/1921138


Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210324164650.128608-1-richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>