folly/wangle/futures/Future-inl.h: avoid -Wsign-compare errors

folly/wangle/futures/Future-inl.h: avoid -Wsign-compare errors

Summary:
* folly/wangle/futures/Future-inl.h (whenAll): Do not compare
signed (std::distance) with unsigned(size_t): add an assertion
to eliminate the possibility of negative, then assign to a new
size_t "n".

Test Plan:
Run this and note there are fewer errors than before:
fbconfig --platform-all=gcc-4.9-glibc-2.20 -r folly && fbmake dbgo

Reviewed By: jsedgwick@fb.com

Subscribers: trunkagent, fugalh, folly-diffs@

FB internal diff: D1770668

Tasks: 5941250

Signature: t1:1770668:1420736965:fc722657a2ec523052fd96c33d65d5ca24aae61c