Fix spurious gcc warning in cc_file.c

gcc 4.6.3 (present in Ubuntu 12.04) is smart enough to look at
get_size and see that it does not always assign to *size_out, but not
smart enough to figure out that it always assigns to *size_out when it
returns 0.  As a result, it outputs two warnings which we treat as
errors.  Add an initial assignment to *size_out at the beginning of
get_size to work around this.

ticket: 8026