sim/cgen: mask uninitialized variable warning in cgen-run.c

I see an uninitialized variable warning (with gcc 9.3.1) from
cgen-run.c, like this:

  /tmp/build/sim/../../src/sim/cris/../common/cgen-run.c: In function ‘sim_resume’:
  /tmp/build/sim/../../src/sim/cris/../common/cgen-run.c:259:5: warning: ‘engine_fns$’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    259 |    (* engine_fns[next_cpu_nr]) (cpu);
        |    ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /tmp/build/sim/../../src/sim/cris/../common/cgen-run.c:232:14: note: ‘engine_fns$’ was declared here
    232 |   ENGINE_FN *engine_fns[MAX_NR_PROCESSORS];
        |              ^~~~~~~~~~

This is a false positive - we over allocate engine_fn, and then only
initialize the nr_cpus entries which we will later go on to use.

However, we can easily silence this warning by initializing the unused
entries in engine_fns to NULL, this might also help if anyone ever
looks at engine_fns in a debugger, it should now be obvious which
entries are in use, and which are not.

With this change the warning is gone.

There should be no change in behaviour with this commit.