Avoid dereferencing null pointer
Avoid dereferencing null pointer

Summary:
Right now in `sendStreamReply`, the pointer `stream` could be null sometimes. Dereferencing this has led to segfaults in my server streaming code.

Seems like D18950357 removed some SCOPE_EXITs which checked nullity of stream

Reviewed By: andriigrynenko

Differential Revision: D19503231

fbshipit-source-id: c1025982bfaa31955d20f20ad25f04daa132fe03

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