memcached_stat_free() invalid read when 0 servers

This applies to libmemcached 0.40.

When I run memcached_stat() without any servers, stat succeeds but memcached_stat_free() crashes on invalid read in stats.c:479 (memc_stat->root). This happens because calloc does not necessarily return NULL on allocation of 0 elements. Simple fixes is to check the errno or always allocate one extra byte.