core: Bad read of file size over TFTP

A fancy pointers logic has been replaced with a plain old if / else
branches. It was assigning only half of a 64 bits integer which is then
assigned to a size_t. Thus leading to a bug on platform where size_t is
64 bits.

Resolves bug #26

Signed-off-by: Celelibi <celelibi@gmail.com>