Unexpected results from f32_sub

[Inokinoki]

Subtraction cannot handle "0" cases:

assert_eq!(crate::soft_f32::f32_sub(0x0, 0xBDCCCCCE), 0x3DCCCCCE);
assert_eq!(crate::soft_f32::f32_sub(0x0, 0x3DCCCCCE), 0xBDCCCCCE);
assert_eq!(crate::soft_f32::f32_sub(0x0, 0x0), 0x0);
assert_eq!(crate::soft_f32::f32_sub(0x0, 0x80000000), 0x0);