libutil: don't memset 64k in drainFD

this is not needed and introduces a bunch of memset calls, making up for
3% of valgrind cycle estimation *alone*. real-world impact is a lot
lower on our test machine, but we suspect that less powerful machines
would see an impact from dropping this.

Change-Id: Iad10e9d556e64fdeb0bee0059a4e52520058d11e

src/libutil/util.cc

@@ -745,7 +745,7 @@ void drainFD(int fd, Sink & sink, bool block)
         }
     });
 
-    std::vector<unsigned char> buf(64 * 1024);
+    std::array<unsigned char, 64 * 1024> buf;
     while (1) {
         checkInterrupt();
         ssize_t rd = read(fd, buf.data(), buf.size());