Fix wchar_t cast

(First noted in http://www.mail-archive.com/fish-users@lists.sourceforge.net/msg01751.html)

Fix a wchar_t cast; grep'd the source for more -- and found that builtin_commandline.cpp has been previously fixed.

The grep expression was rather crude: grep -B2 -- -1 *.[ch]* | grep -e wchar -e ==