In sbflush(), don't exit the while loop too early: this can cause

In sbflush(), don't exit the while loop too early: this can cause
an empty mbuf to stay in the queue, then causing a needless panic
because sb_cc == 0 and sb_mbcnt != 0.

But we still need to panic rather than endlessly looping if, for
some reason, sb_cc == 0 and there are non-empty mbufs in the queue.

PR:		kern/11988
Reviewed by:	fenner