Fix a SEGV in the register allocator.
Fix a SEGV in the register allocator.

This occasionally caused the regress-806 unit test to crash.

Review URL: http://codereview.chromium.org/5913002

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@6043 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh