httplive: avoid zero byte read request

- In LiveSession, check whether maxBytesToRead is zero, if yes
  ignore it because zero byte read request is meaningless.

- In PlaylistFetcher, check whether the read bytes is less than
  the requested block size, if yes, finish fetching the current
  segment file because it means fetching file already meets EOS.

Change-Id: I150d4b147090dbd48d804ebd96ea909e5b6fdbcb