Crash in `cell_is_active_stars()`

The latest master is broken if I try this:

./configure --with-metis --enable-debugging-checks --disable-vec --enable-debug

and run

mpirun -np 4 ../swift_mpi -e -s -t 2 -n 64 sodShock.yml

The code crashes with

[0007] [00000.2] active.h:cell_is_active_stars():210: cell in an impossible time-zone! c->ti_end_min=-1 (t=-1.387779e-18) and e->ti_current=0 (t=0.000000e+00, a=1.000000e+00)

called from the first engine_marktasks() in the initial rebuild.

This is one of the tests run by the automated suite.

I have pushed a temporary fix adc787bc to make the code run but this is dirty and might be the sign that other things are broken. It could be that some cell values are not initialised correctly in the case where there are no stars.