aggregator: Make parsing of explicit sink pad names more robust

When passing "sink_%d" twice to aggregator before it would create two
pads called "sink_0", because it failed to parse "%d" as integer and
used 0 instead then.

Instead validate that parsing was actually successful and also don't
even try to parse if the requested pad name contains a '%'.
21 jobs for backport-mr-202-into-1.16-aggregator-pad-names in 56 minutes and 45 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Preparation
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gst indent

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passed #488733
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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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 #488738
build static nodebug fedora x86_64

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  Test
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check fedora

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integration testsuites fedora

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valgrind bad

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valgrind base

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valgrind core

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valgrind ges

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valgrind good

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  Integrate
passed #488753
android universal examples

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