fdReady(): Fix some C -Wconversion warnings.
fdReady(): Fix some C -Wconversion warnings.

Btw, -Wconversion is off by default and not included in -Wall, -Wextra
or -pedantic, so I used it temporarily with -optc-Wconversion.

Reviewers: bgamari, austin, hvr

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3965

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc