[boost] [pool] patch to make pool support zero sized allocations.

AMDG The patch at https://svn.boost.org/trac/boost/attachment/ticket/386/pool.patch allows the pool classes to support zero sized allocation, by returning a null pointer. std::vector sometimes allocates a zero sized block, so this is necessary for pool_allocator. Returning a null pointer is the simplest solution and I'm that the standard allows an allocator to return the null pointer when asked for a block of zero elements. Okay to commit? In Christ, Steven Watanabe