Fastbufs: bclose() on caller-allocated fastbuf is allowed

Fastbufs: bclose() on caller-allocated fastbuf is allowed

Previously, it was impossible to catch exceptions thrown by fbbuf
(although it is quite useful in case of the "buffer overflow" exception).

With these changes, bclose() should be allowed and equivalent to bflush(),
which should do nothing :)  Hence, fb_tie() should work and so should
exception handling.