openni_pcd_recorder: Fix memory size calculation for 32-bit system with >4GB RAM.

Systems with e.g. a Linux PAE kernel can have more than 4GB RAM, which
means the size should always be calculated using 64-bit integers.  The
memory available for buffering, however, must be addressable by a
std::size_t.
Merge pull request #676 from j-oberlaender/master

openni_pcd_recorder: Fix memory size calculation for 32-bit system with ...