tests: Use ZERO_STRUCT for readfds

This fixes a compiler warning on FreeBSD.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 3b896750b8)

tests/unittests/torture_channel.c

@@ -12,13 +12,15 @@
 
 static void torture_channel_select(void **state)
 {
-    fd_set readfds = {0};
+    fd_set readfds;
     int fd;
     int rc;
     int i;
 
     (void)state; /* unused */
 
+    ZERO_STRUCT(readfds);
+
     fd = open("/dev/null", 0);
     assert_true(fd > 2);