JVET-AE0142 item 3 and item 4: Fix two bugs including an added condition and a nnpfc_purpose related constraint.

Wei Jiarequested to merge
wei/VVCSoftware_VTM:JVET_AE0142 into master

JVET_AE0142 item 3 and item 4: Fix two bugs including an added condition and a nnpfc_purpose related constraint.

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