BQ: Fix segfault in dump()

When dequeueBuffer() is called we put the slot in mActiveBuffers, then
don't hold the BufferQueue lock while allocation occurs. So a slot
might be in mActiveBuffers but not have a buffer attached yet. Prevent
the dump function from segfaulting in this case.

Bug 27128710

Change-Id: Ie1480c0f9b2544554fc3281045e55ad30605e6ec