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 '%'.
23 jobs for aggregator-pad-names in 46 minutes and 28 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #408427
gst indent

00:00:43

passed #408426
manifest

00:00:27

 
  Build
passed #408433
build cerbero android universal

00:11:54

passed #408434
build cerbero cross win32

00:08:07

passed #408435
build cerbero cross win64

00:06:16

passed #408432
build cerbero fedora x86_64

00:05:52

manual #408437
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #408436
gst-macos-10.14
build cerbero macos x86_64

00:07:18

passed #408428
build fedora x86_64

00:08:46

passed #408429
build nodebug fedora x86_64

00:06:57

passed #408430
build static fedora x86_64

00:13:03

passed #408431
build static nodebug fedora x86_64

00:11:39

 
  Test
passed #408584
check fedora

00:03:33

passed #408439
integration testsuites fedora

00:21:57

passed #408444
valgrind bad

00:08:18

passed #408441
valgrind base

00:14:43

passed #408440
valgrind core

00:17:59

passed #408445
valgrind ges

00:04:22

passed #408442
valgrind good

00:16:24

passed #408443
valgrind ugly

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failed #408438
check fedora

00:04:21

 
  Integrate
passed #408446
android universal examples

00:07:08

passed #408447
documentation

00:05:07