bin: Fix minor race when adding to a bin

This patch simply add a null check around a case where a child may have
been unparented concurrently to the deep_add_remove operation. This was
found by accident in the form of an "IS_GST_OBJECT" assertion, but had
no other known side effect in that test.
21 jobs for backport-mr-271-into-1.16-bin-depp-add-race in 59 minutes and 45 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #582785
gst indent

00:00:34

passed #582784
manifest

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  Build
passed #582791
build cerbero android universal

00:10:16

passed #582792
build cerbero cross win32

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passed #582793
build cerbero cross win64

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passed #582790
build cerbero fedora x86_64

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passed #582795
gst-ios-12.2
build cerbero ios universal

00:37:53

passed #582794
gst-macos-10.14
build cerbero macos x86_64

00:09:36

passed #582786
build fedora x86_64

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passed #582787
build nodebug fedora x86_64

00:07:22

passed #582788
build static fedora x86_64

00:11:12

passed #582789
build static nodebug fedora x86_64

00:10:42

 
  Test
passed #582796
check fedora

00:05:24

passed #582797
integration testsuites fedora

00:13:00

passed #582802
valgrind bad

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passed #582799
valgrind base

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passed #582798
valgrind core

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passed #582803
valgrind ges

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passed #582800
valgrind good

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passed #582801
valgrind ugly

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  Integrate
passed #582804
android universal examples

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