audio: use gst_buffer_extract() in copyGstreamerBuffersToAudioChannel()

copyGstreamerBuffersToAudioChannel() was mapping the GstBuffer content to memcpy
it to the AudioChannel buffer. This patch leverages gst_buffer_extract() to
simplify the code as it does exactly what we need: gst_buffer_map, memcpy,
gst_buffer_unmap and error handling.

Also replace GstBuffer NULL check by an assertion as we already make sure they
are not NULL before adding them to the list.

Original patch by: Christophe Dumez  <ch.dumez@sisa.samsung.com>
Reference: https://bugs.webkit.org/show_bug.cgi?id=113880

Reviewed by: Hugo Lima