Fetch all results in php_memc_get_impl

Previously only the first result was fetched after the
memcached_mget_by_key call resulting in a RES_END result being left on
the stack. As per the libmemcached documentation memcached_fetch_result
should be called until it returns NULL following a multi-get.

In some circumstances this is not noticeable as libmemcached takes care
of flushing the receive buffers accordingly, however certain subsequent
operations will incorrectly return the left-over RES_END result.