serial: clear serial info struct before calling ioctl()

We need the kernel driver to give a proper value for the 'closing_wait'
variable, but don't assume it will.

This should solve the following kind of issues reported by valgrind:

==8985== Conditional jump or move depends on uninitialised value(s)
==8985==    at 0x4409A6: mm_serial_port_close (mm-serial-port.c:932)
==8985==    by 0x41A420: disable_all_done (mm-generic-cdma.c:753)
==8985==    by 0x4125A4: mm_manager_shutdown (mm-manager.c:1130)
==8985==    by 0x40DE35: main (main.c:203)

1 files changed, 2 insertions, 2 deletions

diff --git a/src/mm-serial-port.c b/src/mm-serial-port.c
index 0a8820d2..04cac88c 100644
--- a/src/mm-serial-port.c
+++ b/src/mm-serial-port.c
@@ -840,7 +840,7 @@ mm_serial_port_open (MMSerialPort *self, GError **error)
     MMSerialPortPrivate *priv;
     char *devfile;
     const char *device;
-    struct serial_struct sinfo;
+    struct serial_struct sinfo = { 0 };
     GTimeVal tv_start, tv_end;
 
     g_return_val_if_fail (MM_IS_SERIAL_PORT (self), FALSE);
@@ -977,7 +977,7 @@ mm_serial_port_close (MMSerialPort *self)
 
     if (priv->fd >= 0) {
         GTimeVal tv_start, tv_end;
-        struct serial_struct sinfo;
+        struct serial_struct sinfo = { 0 };
 
         mm_info ("(%s) closing serial port...", device);