audio: fix wrong ASSERT in AudioDestination::stop()

Correct erroneous ASSERT, we want both member variables to exist.

Patch by Xan Lopez <xlopez@igalia.com>
Reference: https://bugs.webkit.org/show_bug.cgi?id=112344

Reviewed by: Hugo Lima
audio: g_slist_reverse() may not be required in webKitWebAudioSrcLoop

Decremental 'for' loop logic implemented to avoid using g_slist_reverse().
    Original code            - 2.025230 micro seconds per loop
    Original code + patch    - 1.964759 micro seconds per loop

Patch by Praveen R Jadhav  <praveen.j@samsung.com>
Reference: https://bugs.webkit.org/show_bug.cgi?id=113568

Reviewed by: Jesus Sanchez-Palencia