sim/ppc: initialize a memory buffer in all cases

sim/ppc: initialize a memory buffer in all cases

In the ppc simulator's do_fstat function, which provides the fstat
call for the simulator, if the fstat is going to fail then we
currently write an uninitialized buffer into the simulated target.

In theory, I think this is fine, we also write the error status into
the simulated target, so, given that the fstat has failed, the target
shouldn't be relying on the buffer contents.

However, writing an uninitialized buffer means we might leak simulator
private data into the simulated target, which is probably a bad thing.
Plus it probably makes life easier if something consistent, like all
zeros, is written rather than random junk, which might look like a
successful call (except for the error code).

So, in this commit, I initialize the stat buffer to zero before
it is potentially used.  If the stat call is not made then the buffer
will be left initialized as all zeros.