daemon: Fix crash in virTypedParameterArrayClear

daemon: Fix crash in virTypedParameterArrayClear

Daemon uses the following pattern when dispatching APIs with typed
parameters:

    VIR_ALLOC_N(params, nparams);
    virDomain*(dom, params, &nparams, flags);
    virTypedParameterArrayClear(params, nparams);

In case nparams was originally set to 0, virDomain* API would fill it
with the number of typed parameters it can provide and we would use this
number (rather than zero) to clear params. Because VIR_ALLOC* returns
non-NULL pointer even if size is 0, the code would end up walking
through random memory. If we were lucky enough and the memory contained
7 (VIR_TYPED_PARAM_STRING) at the right place, we would try to free a
random pointer and crash.

Let's make sure params stays NULL when nparams is 0.