Fix access to data that might be deleted

PPS data in reference pictures are deleted if a PPS with the same
PPS_id have been sent, and the access to getMixedNaluTypesInPicFlag() is
not valid any more.
6 jobs for !1968 with fix_access_to_deleted_data in 16 minutes and 8 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Build
passed #19601
macos
build_macos

00:04:36

passed #19602
ubuntu1604
build_ubuntu1604

00:03:28

passed #19603
ubuntu1804
build_ubuntu1804

00:03:49

passed #19604
ubuntu1804-gcc8
build_ubuntu1804-gcc8

00:04:24

passed #19605
vc191x
build_vc191x

00:16:05

passed #19606
vc192x
build_vc192x

00:13:16