Whenever there is an overflow in code_gen_buffer (e.g. we run out
of space in it and have to flush it), the code_time profiling counter
ends up with an invalid value (that is, code_time -= profile_getclock(),
without later on getting += profile_getclock() due to the goto).
Fix it by using the ti variable, so that we only update code_time
when there is no overflow. Note that in case there is an overflow
we fail to account for the elapsed coding time, but this is quite rare
so we can probably live with it.