asset: Do not take an extra ref on asset when already initialized

The task already has a ref so this one doesn't make sense and leads to leaks
11 jobs for ges_valgrind in 38 minutes and 41 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #241712
gst indent

00:00:13

passed #241711
manifest

00:00:37

 
  Build
passed #241713
build fedora x86_64

00:05:21

passed #241714
build nodebug fedora x86_64

00:05:19

 
  Test
passed #241715
check fedora

00:03:06

passed #241716
integration testsuites fedora

00:15:05

 
  Full Builds
passed #241718
build cerbero android universal

00:10:28

passed #241719
build cerbero cross win32

00:07:15

passed #241720
build cerbero cross win64

00:06:14

passed #241717
build cerbero fedora x86_64

00:05:43

 
  Apps
passed #241721
android universal examples

00:07:06