Fix off-by-one in TestPagebufferReader2

[andhe]

Fixes #1197

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This change is 

[claassistantio]

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[coveralls]

Coverage increased (+0.1%) to 69.78% when pulling 1bb7749 on andhe:fix-eof-test into 3e25d77 on dgraph-io:master.

[jarifibrahim]

@andhe can you please sign the CLA?

[andhe]

Sorry for the trouble, but I'm not signing any CLAs.

There's absolutely no way my change is even copyrightable, so this should not be a problem for you. Even Free Software Foundation is able to take trivial patches without paperwork. (Do you really want to be that organisation with the "more anal than the FSF" slogan?). If you aren't able to handle this, please fell free to use your full creativity to reimplement "- 1". ;)

[jarifibrahim]

Thank you @andhe for the feedback but because of the repository settings, I can't merge any PRs without the CLA signed. Thank you again for your contribution. I'll close this PR. :)