Backport of "appsink: add missing make_writable call" into 1.22
requested to merge tpm/gstreamer:mr-4991-backport-into-1.22-appsink-add-missing-make-writable into 1.22
Backport of !4991 (merged) "appsink: add missing make_writable call".
Commits:
- 83173712 appsink: add missing make_writable call