Change BUG() messages in buf_flush_to_tls() to IF_BUG_ONCE()

We introduced these BUG() checks in b0ddaac0 to prevent a
recurrence of bug 23690.  But there's a report of the BUG() message
getting triggered and filling up the disk.  Let's change it to
IF_BUG_ONCE().

Fixes bug 33093; bugfix on 0.3.2.2-alpha.