Fix compile warning in if_fxp(4).

[markj]

Summary

clang 7 warns about the negative array index. Try to implement the same
logic without invoking undefined behaviour.

Test Plan

There is no difference in the disassembly before and after this change.

[imp]

This matches what I puzzled out the right fix should be, and is a bit better than what I had in mind before this problem slipped away...

[brooks]

Thanks for chasing this down. This was quite the abomination...