journal: replace a bunch of assert() with friendlier checks

journal: replace a bunch of assert() with friendlier checks

We should not rely that data stored in the journal files remains
entirely untouched at all times. Because we unallocate files, data might
go away any time. Hence, never assert() on any expectations on what the
file contains. Instead, handle it more gracefully as a corruption issue,
and return EBADMSG.

Fixes: #35229 #32436