fix segfault in redexdump
fix segfault in redexdump

Summary:
how did this code ever not segfault?!

See https://github.com/facebook/redex/issues/225

Reviewed By: int3

Differential Revision: D5294964

fbshipit-source-id: 5b12ebacb8c9e74bccbfbd1556251aa91ce15595

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex