Fix HttpStreamParser fuzzer.
Fix HttpStreamParser fuzzer.

It was failing to read the body.  0 while reading the headers means OK,
while 0 while reading the body means the connection was closed. I fixed
this issue two weeks ago, but then re-introduced it in a refactor.

I think that shows how weird and confusing network stack result handling
is.  :(

BUG=598037

Review URL: https://codereview.chromium.org/1920463003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#389314}

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