when decompressing zlib compressed packets, use Z_SYNC_FLUSH instead

when decompressing zlib compressed packets, use Z_SYNC_FLUSH instead
of Z_PARTIAL_FLUSH as the latter is not actually specified as a valid
mode for inflate(). There should be no practical change in behaviour
as the compression side ensures a flush that should make all data
available to the receiver in all cases.

repoted by lamm AT ibm.com via bz3372; ok markus