storage: fix unlikely memory leak in rbd backend

storage: fix unlikely memory leak in rbd backend

virStorageBackendRBDRefreshPool() first allocates an array big enough
to hold 1024 names, then calls rbd_list(), which returns ERANGE if the
array isn't big enough. When that happens, the VIR_ALLOC_N is called
again with a larger size. Unfortunately, the original array isn't
freed before allocating a new one.