Fix incorrect use of IOBuf::reserve
Fix incorrect use of IOBuf::reserve

Differential Revision: D2037078

fb-gh-sync-id: d8bc304506ea70bfde3a3490f1aac23bc4392efa
shipit-source-id: d8bc304506ea70bfde3a3490f1aac23bc4392efa

apaszke/thpp,apaszke/thpp