Fix MSVC warning on streampos to size_t conversion

Fix this warning with MSVC 64 bits:

warning C4244: '=' : conversion from 'std::streampos' to 'size_t',
possible loss of data

Point is that std::streampos could be negative, while size_t
is always non-negative.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>