It seems img_auth.php has been broken in the REL1_31, REL1_32 and REL1_33 branches because https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/496705/ has been merged to them without also merging https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/505351/ that it relies on.
So any time wfImageAuthMain tries to call HTTPFileStreamer::preprocessHeaders an exception is thrown as that function does not exist.