ubsan: thrift: fix cast in compiler
ubsan: thrift: fix cast in compiler

Summary:
```
 |=> 23 MORE THREADS: ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] ['] [']
 thrift/compiler/generate/t_cpp_generator.cc:1361:22: runtime error: downcast of address 0x00000103cbf0 which does not point to an object of type 't_list'
 0x00000103cbf0: note: object is of type 't_set'
  00 00 00 00  b0 56 15 be 87 7f 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00
                ^~~~~~~~~~~~~~~~~~~~~~~
                              vptr for 't_set'
                              ```

Reviewed By: andrewjcg

Differential Revision: D3283349

fbshipit-source-id: 79c466ef167eb7165a224f1df8a96ac911a18e5b

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