conf: avoid freeing network object with undestroyed mutex

conf: avoid freeing network object with undestroyed mutex

virNetworkAssignDef was allocating a new network object, initing and
grabbing its lock, then potentially freeing it without unlocking or
destroying the lock. In practice 1) this will probably never happen,
and 2) even if it did, the lock implementation used on most (all?)
platforms doesn't actually hold any resources for an initialized or
held lock, but it still bothered me, so I moved the realloc that could
lead to this bad situation earlier in the function, and now the mutex
isn't inited or locked until we are assured of complete success.