Fix a potential segfault in phpiredis_command_bs() when the Redis server goes away

The error occurred when the connection to the redis server was lost during a command. In that case the pointer to the reply object, which was uninitialized, would be freed, which leads to a segfault.

This pull request also fixes some possible memory leaks in `phpiredis_multi_command()` and `phpiredis_multi_command_bs()`.