Field splitting regression on master branch

As of commit c0674f487c7aec2a3bdf6795cea7e60c9530c360, field splitting is 
broken. Minimal-ish reproducer:

$ dash -c 'IFS=:; v=" :: :: "; set $v; echo $#'
5

Expected output: 2

FYI, this is a nice cross-platform script for testing for field splitting 
regressions (on dash as well):

http://web.archive.org/web/20051225040851/http://www.research.att.com/~gsf/public/ifs.sh

Dash passes this prior to the referenced commit, and fails it after, in 
informative ways.

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