coverity: ftell returns a signed value

The check for flen < 0 could never have been true since flen was declared
to be size_t (unsigned). Declare flen to be long since that is what ftell
returns.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>