interlace: Reset after changing state to READY

Trying to reset before the pads have been deactivated races with the
streaming thread. There was also a buggy buffer clear leaving a dangling
`stored_frame` pointer around. Use `gst_interlace_reset` so this happens
properly.

Part-of: <gstreamer/gst-plugins-bad!2553>
29 jobs for !2553 with interlace-double-unref-1.18 in 39 minutes and 24 seconds (queued for 11 seconds)
detached
Status Name Job ID Coverage
  Build Docker
passed alpine amd64 manifest builder docker #15017753

00:01:22

passed android docker #15017752

00:07:12

passed cerbero fedora amd64 docker #15017751

00:11:26

passed fedora amd64 docker #15017750

00:12:09

passed gst-indent amd64 docker #15017754

00:00:30

 
  Preparation
passed gst indent #15017756

00:00:35

passed manifest #15017755
manual

00:00:48

 
  Build
passed build cerbero cross-android universal #15017765
gstreamer

00:14:39

passed build cerbero cross-ios universal #15017769
gst-ios-14.3

00:19:41

passed build cerbero cross win32 #15017766
gstreamer

00:11:55

passed build cerbero cross win64 #15017767
gstreamer

00:12:09

passed build cerbero fedora x86_64 #15017764
gstreamer

00:09:57

passed build cerbero macos x86_64 #15017768
gst-macos-11.1

00:10:27

passed build clang fedora x86_64 #15017760

00:18:36

manual build msys2 #15017763
docker windows 1809 allowed to fail manual
passed build nodebug fedora x86_64 #15017757

00:17:57

passed build static fedora x86_64 #15017758

00:17:36

passed build static nodebug fedora x86_64 #15017759

00:12:16

passed build vs2017 amd64 #15017761
docker windows 1809

00:10:01

passed build vs2017 x86 #15017762
docker windows 1809

00:10:11

 
  Test
passed check fedora #15017770
gstreamer

00:11:43

passed integration testsuites fedora 1/4 #15017771
gstreamer

00:13:03

passed integration testsuites fedora 2/4 #15017772
gstreamer

00:12:28

passed integration testsuites fedora 3/4 #15017773
gstreamer

00:04:27

passed integration testsuites fedora 4/4 #15017774
gstreamer

00:05:56

passed valgrind bad #15017775
gstreamer

00:04:15

 
  Integrate
passed cross-android universal examples #15017776

00:10:56

passed cross-ios universal examples #15017778
gst-ios-14.3

00:04:35

passed documentation #15017777

00:13:08