guard negative value_size in memcache PopStore

[ubeddulla]

MemcacheResponse::PopStore derives value_size by subtracting the reply's extras_length and key_length from total_body_length, but unlike its siblings PopGet, PopCounter and PopVersion it never checks whether value_size went negative before handing it to cutn(&_err, value_size). A server (or man-in-the-middle) that returns a STORE/DELETE/FLUSH error reply whose extras plus key exceed the body drives value_size below zero, so the huge size_t drains the rest of the pipelined buffer into the error string and desyncs the following replies on the connection. Add the same value_size < 0 guard the sibling parsers already carry.