vl.c: Don't zero-initialize statics for serial_hds

checkpatch reminds us that statics shouldn't be zero-initialized:

ERROR: do not initialise statics to 0 or NULL
#35: FILE: vl.c:157:
+static int num_serial_hds = 0;

ERROR: do not initialise statics to 0 or NULL
#36: FILE: vl.c:158:
+static Chardev **serial_hds = NULL;

I forgot to fix this in 6af2692e; do so now.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Message-id: 20180426140253.3918-1-peter.maydell@linaro.org

vl.c

@@ -151,8 +151,8 @@ QEMUClockType rtc_clock;
 int vga_interface_type = VGA_NONE;
 static DisplayOptions dpy;
 int no_frame;
-static int num_serial_hds = 0;
-static Chardev **serial_hds = NULL;
+static int num_serial_hds;
+static Chardev **serial_hds;
 Chardev *parallel_hds[MAX_PARALLEL_PORTS];
 Chardev *virtcon_hds[MAX_VIRTIO_CONSOLES];
 int win2k_install_hack = 0;