ass: avoid infinite unref loop with bad data

A classic case of not updating the next item to iterate after deleting
it from the singly linked list.

Only ever hit with a text buffer with GST_CLOCK_TIME_NONE for either the
timestamp or duration.
21 jobs for master in 109 minutes and 10 seconds (queued for 17 seconds)
Status Job ID Name Coverage
  Preparation
passed #702266
gst indent

00:00:38

passed #702265
manifest

00:00:24

 
  Build
passed #702276
build cerbero cross-android universal

00:18:02

manual #702280
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #702277
build cerbero cross win32

00:09:27

passed #702278
build cerbero cross win64

00:06:59

passed #702275
build cerbero fedora x86_64

00:07:03

passed #702279
gst-macos-10.14
build cerbero macos x86_64

00:07:07

passed #702271
build clang fedora x86_64

00:08:28

passed #702267
build fedora x86_64

00:06:19

passed #702274
docker windows 1607
build msys2

00:13:01

passed #702268
build nodebug fedora x86_64

00:03:33

passed #702269
build static fedora x86_64

00:13:11

passed #702270
build static nodebug fedora x86_64

00:04:46

passed #702272
docker windows 1607
build vs2017 amd64

00:09:30

passed #702273
docker windows 1607
build vs2017 x86

00:10:16

 
  Test
passed #702281
check fedora

00:03:54

failed #702282
integration testsuites fedora

01:38:30

 
  Integrate
passed #702283
cross-android universal examples

00:09:29

skipped #702285
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #702284
documentation

00:06:23

 
Name Stage Failure
failed
integration testsuites fedora Test
  |  #1  0x00007f60ad576e33 in g_cond_wait (cond=cond@entry=0x17acbf0, mutex=mutex@entry=0x17acba8) at ../glib/gthread-posix.c:1413
| #2 0x00007f60ad6d1957 in gst_task_func (task=0x17acb90 [GstTask]) at ../subprojects/gstreamer/gst/gsttask.c:313
| #3 0x00007f60ad554f14 in g_thread_pool_thread_proxy (data=<optimized out>) at ../glib/gthreadpool.c:308
| #4 0x00007f60ad5544e2 in g_thread_proxy (data=0x7f60a0033ca0) at ../glib/gthread.c:805
| #5 0x00007f60acde95a2 in start_thread (arg=<optimized out>) at pthread_create.c:486
| #6 0x00007f60ad1de163 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
|
| Thread 3 (Thread 0x7f60aa086700 (LWP 10838)):
| Job's log exceeded limit of 4194304 bytes.