lang: Fix memory corruption bug introduced in commit ac613331d5062bcd1aebc25e03c7f321296c4110

- shifts from pre to post increment/decrement created a subtle bug

Signed-off-by: Scott Wilson <i@scottwilson.ca>
Merge pull request #1054 from supercollider/fix-corruption

lang: Fix memory corruption bug introduced in commit ac613331d5062bcd1ae...