Fix places where a negative return from ftello could be used as invalid input

Fix places where a negative return from ftello could be used as invalid input

In a variety of locations in both reading and writing a file, the result
from the C library function ftello is used as input to other functions.  For
the parameters and functions in question, a negative value is invalid input.
This patch checks the return value from the ftello function to determine if
we were able to determine the current position in the file stream and, if not,
fail gracefully.

(issue ASTERISK-19655)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/1863/
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Merged revisions 362355 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@362356 65c4cc65-6c06-0410-ace0-fbb531ad65f3