target/m68k: fix set_cc_op()

The first call of set_cc_op() in a new translation sequence
is done with old_op set to CC_OP_DYNAMIC (-1).

This will do an out of bound access to the array cc_op_live[].

We fix that by adding an entry in cc_op_live[] for CC_OP_DYNAMIC.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20171221160558.14151-1-laurent@vivier.eu>