Blender Git Loki

Fix Crash: Loading Huge Images.

When loading huge images (30k) blender crashed with a buffer overflow.
The reason is that determine the length of a buffer was done in 32bit
precision and afterwards stored in 64 bit precision.

This patch adds a new function to do the correct calculation in 64bit.
It should be added to other sections in blender as well. But that should
be tested on a per case situation.