vtest: Initialize sendmsg iovec contents

To keep Valgrind happy:

==3628== Syscall param sendmsg(msg.msg_iov[0]) points to uninitialised byte(s)
==3628==    at 0x48DB431: sendmsg (sendmsg.c:28)
==3628==    by 0x10C00E: vtest_create_resource2 (in /virglrenderer/build/vtest/virgl_test_server)
==3628==    by 0x10AC2B: vtest_server_run (in /virglrenderer/build/vtest/virgl_test_server)
==3628==    by 0x10A5B3: main (in /virglrenderer/build/vtest/virgl_test_server)
==3628==  Address 0x1fff0004ef is on thread 1's stack
==3628==  in frame #1, created by vtest_create_resource2 (???:)

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>

vtest/vtest_renderer.c

@@ -193,7 +193,8 @@ int vtest_block_read(struct vtest_input *input, void *buf, int size)
 static int vtest_send_fd(int socket_fd, int fd)
 {
     struct iovec iovec;
-    char buf[CMSG_SPACE(sizeof(int))], c;
+    char buf[CMSG_SPACE(sizeof(int))];
+    char c = 0;
     struct msghdr msgh = { 0 };
     memset(buf, 0, sizeof(buf));