qtdemux: Use empty-array safe way to cleanup GPtrArray

Fix assertion fail
GLib-CRITICAL **: g_ptr_array_remove_range: assertion 'index_ < rarray->len' failed
19 jobs for backport-mr-282-into-1.16-qtdemux-assert in 74 minutes and 52 seconds (queued for 21 seconds)
Status Job ID Name Coverage
  Preparation
passed #517084
gst indent

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passed #517083
manifest

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  Build
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build cerbero android universal

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build cerbero cross win32

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build cerbero cross win64

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build cerbero fedora x86_64

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gst-ios-12.2
build cerbero ios universal

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gst-macos-10.14
build cerbero macos x86_64

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passed #517085
build fedora x86_64

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build nodebug fedora x86_64

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build static fedora x86_64

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passed #517088
build static nodebug fedora x86_64

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  Test
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check fedora

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passed #517309
integration testsuites fedora

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passed #517098
valgrind bad

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valgrind ges

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passed #517097
valgrind good

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canceled #517096
integration testsuites fedora

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  Integrate
passed #517100
android universal examples

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