Conv.h: fix a type error caught by clang

Conv.h: fix a type error caught by clang

Summary: Need to be more explicit about types; it was trying to add types deduced to be `unsigned long` + `long` which didn't agree and could be an overflow.  Be explicit anyway about the types involved.  Appease Clang, and also make the code more obvious to the reader.

Test Plan: Tried with gcc 4.9, clang 3.5, clang 3.6.

Reviewed By: yfeldblum@fb.com

Subscribers: mathieubaudet, maoy, folly-diffs@, yzhan, yfeldblum, chalfant

FB internal diff: D2134814

Tasks: 7337462

Signature: t1:2134814:1433726862:5dd80b198187c610f793e26160919922863a22a2

Blame Revision: D1934777