Remove a use of a negative array index from fxp(4).

This fixes a warning seen when compiling amd64 GENERIC with clang 7.
Also remove the workaround added in r337324. clang 7 and gcc 4.2
generate the same code with or without the code change.

Reviewed by: imp (previous version)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D18603