Truncate set-like size before the negative check

[ethanstoner]

Fixes #1731.

GetSetRecord runs ToIntegerOrInfinity on the size before checking it against zero, so a size in (-1, 0) becomes 0 and is legal. js_setlike_get_props checked the raw number, which threw RangeError for -0.5. Check for NaN first, truncate, then do the negative check, as the spec orders it.

new Set().intersection({ get has() { throw 0; }, size: -0.5 });
// before: RangeError: .size is not a legal size
// after:  0 (thrown by the has getter)

Added tests/bug1731.js. make test passes (0/118 errors) and test262 built-ins/Set is 0/383 errors before and after.