Code failed to allow for QPDF::getCompressibleObjSet deleting objects
from the object cache in case of multiple entries for the same object id.
Add fuzz test case 68668.
Fix two errors introduced in #1110 and #1112. Since
#1110, encountering the invalid indirect reference #1110
-2147483648 n R produces an integer underflow which, if
undetected, immediately trigger a logic error. Since
#1112, object -1 0 R may be incorrectly identified as
an earlier generation of itself and deleted,
invalidating a live iterator.
It is possible to reproduce the failure with this file following the
instructions with oss-fuzz, though it does not cause a failure in CI.
The failure was introduced in
18c1ffe0df.
