FILE: fix retrieval of file contents when offset is specified

The loop that reads in a file was not correctly using the offset when
determining what bytes to append to the output. This patch corrects
the logic such that the correct portion of the file is extracted when an
offset is specified.

ASTERISK-21765
Reported by: John Zhong
Tested by: Matt Jordan, Di-Shi Sun
patches:
  file_read_390821.patch uploaded by Di-Shi Sun (License 5076)
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Merged revisions 432935 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 432938 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432940 65c4cc65-6c06-0410-ace0-fbb531ad65f3