matroskademux: Don't leak allocated index memory

Don't forget to free returned memory from _search_pos()
6 jobs for fix-matroska-leak in 29 minutes and 17 seconds (queued for 2 minutes and 31 seconds)
Status Job ID Name Coverage
  Preparation
passed #69753
gst indent

00:00:36

passed #69752
manifest

00:00:24

 
  Build
passed #69754
build fedora x86_64

00:03:42

 
  Test
passed #69755
check fedora

00:16:28

 
  Full Builds
passed #69756
build android arm64 api28

00:04:47

passed #69757
build cerbero fedora x86_64

00:08:04