The `fmap_readn()` function returns a size_t.
On error, it returns (size_t)-1. The SIS, SWF, and TNEF parsers were
storing the result as a signed int and then checking if < 0 for the
error case.
Added a CLI_ISCONTAINED_2_0_TO() macro, like the CLI_ISCONTAINED_0_TO()
macro for use when the big buffer offset == 0, to eliminate pointless
warnings.
Fix enum return type for functions in SWF parser.
Errors are enums, not ints! There's a difference.
Fix if-check in SWF parser where we relied on integer overflow for error
checking.
