After commit 1ea9735a I see these error while using the webcam
integrated in my laptop:

GStreamer-CRITICAL **: gst_value_list_get_size: assertion 'GST_VALUE_HOLDS_LIST (value)' failed

The issue is gst_v4l2src_value_simplify() was doing its job of
generating a single value, rather than the original list. That why,
when getting the list size, a critical warning was raised.

This patch takes advantage of the compiler optimizations to verify
first if the list was simplified, thus use it directly, otherwise,
if it is a list, verify its size.

https://bugzilla.gnome.org/show_bug.cgi?id=776106