sp-capture-writer: Initialize write buffer

Valgrind notes that the buffer is not fully initialized when we pass it
to write(). This could potentially write sensitive data, if we are not
careful with the buffer length.

See https://gitlab.gnome.org/GNOME/gjs/issues/120

https://bugzilla.gnome.org/show_bug.cgi?id=794272

lib/capture/sp-capture-writer.c

@@ -425,7 +425,7 @@ sp_capture_writer_new_from_fd (int   fd,
   self = g_new0 (SpCaptureWriter, 1);
   self->ref_count = 1;
   self->fd = fd;
-  self->buf = (guint8 *)g_malloc (buffer_size);
+  self->buf = (guint8 *)g_malloc0 (buffer_size);
   self->len = buffer_size;
   self->next_counter_id = 1;