Fix a json_decode crash when depth==0

Summary:
Setting depth=0 is an error, and should result in NULL, but we weren't
checking for it, so in the case of a single, top-level string, we
would reading the -1th element of the stack.

Differential Revision: D19609959

fbshipit-source-id: 04ca1e0965e04b44df2d5c806a73c3da99ff66fb