[Backport r50783 | neal.norwitz. The bytes_left code is complicated,

[Backport r50783 | neal.norwitz.  The bytes_left code is complicated,
 but looks correct on a casual inspection and hasn't been modified
 in the trunk.  Does anyone want to review further?]

Ensure we don't write beyond errText.  I think I got this right, but
it definitely could use some review to ensure I'm not off by one
and there's no possible overflow/wrap-around of bytes_left.
Reported by Klocwork #1.

Fix a problem if there is a failure allocating self->db.
Found with failmalloc.