liblog: remove memset() before recv().

This is unneeded, since we're already checking the length returned by
recv() and log_msg that is read for validity.

It costs ~4% of CPU with `logcat -s` and ~2% of CPU when running
simpleperf for 1 second on walleye on master.

Bug: 144311420
Test: logcat works, simpleperf doesn't show memset() costing as much.
Change-Id: I986e7e96518774034340f1b1201a2071a904e3bb