expr: fix memory management

[E5ten]

Andy Chu's patch from the mailing list, modified slightly to fit the current source. http://lists.landley.net/htdig.cgi/toybox-landley.net/2016-March/008113.html

This fixes #147 as well as fixing all remaining memory management issues in expr from what I can see (valgrind now gives no warnings or any indication of issues while previously it reported many invalid reads).

[E5ten]

ping

[E5ten]

uh I did not mean to do that

[enh-google]

(you might want to squash the two commits into one for ease of review. you probably also want to just send the patch on the list, because rob pays more attention to that than to the pull requests...)