It is not possible unfortunately for memcached decrement unfortunately.
But you could use subdocument counters (decrement fields inside the document), it behaves just like regular counters, but allows you to dive into document structure. Also it does allow negative values, and gives you error on overflow (smaller than -9223372036854775808 or larger than 9223372036854775807). For example:
$ php test.php
cannot increment 'cnt': LCB_SUBDOC_MULTI_FAILURE: Could not execute one or more multi lookups or mutations