s/size_t/off_t/ in sendTimeLineHistory

Hi,

When looking at the report of probable fallout from the elog stuff I
noticed this warning:

walsender.c(366): warning #186: pointless comparison of unsigned integer with zero

Which is accurate, the rather unlikely problem of a seek error could not
be noticed atm and would probably send strange stuff over the wire.

Greetings,

Andres Freund

--
 Andres Freund                       http://www.2ndQuadrant.com/
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