Crash running benchmarks game's binary trees bench

[Charles Oliver Nutter]

FYI, this produced a SIGSEGV:

jruby --server bench/shootout/binarytrees.ruby-2.ruby 15

14 and lower worked ok. Crash is consistent. Running macosx build from
4/28...not sure what build of mainline it maps to...

~/projects/jruby ➔ java -version
openjdk version "1.7.0-internal"
OpenJDK Runtime Environment (build 1.7.0-internal-b00)
OpenJDK 64-Bit Server VM (build 21.0-b08, mixed mode)

- Charlie