chardev: fix incorrect unref of source

glib reported error when pty chardev used:

$ ./qemu-system-x86_64 -chardev pty,id=foo -device isa-serial,chardev=foo
qemu-system-x86_64: -chardev pty,id=foo: char device redirected to /dev/pts/2 (label foo)
(qemu-system-x86_64:27885): GLib-CRITICAL **: g_source_unref: assertion 'source != NULL' failed
(qemu-system-x86_64:27885): GLib-CRITICAL **: g_source_unref: assertion 'source != NULL' failed

This patch fixes that.

Fixes: 2c716ba1 ("chardev: introduce qemu_chr_timeout_add_ms()")
CC: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180118052049.31119-1-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>