Fix an issue where a caller to ast_write on a MulticastRTP channel would...

Fix an issue where a caller to ast_write on a MulticastRTP channel would determine it failed when in reality it did not.

When sending RTP packets via multicast the amount of data sent is stored in a variable and returned
from the write function. This is incorrect as any non-zero value returned is considered a failure while
a return value of 0 is success. For callers (such as ast_streamfile) that checked the return value
they would have considered it a failure when in reality nothing went wrong and it was actually a success.

The write function for the multicast RTP engine now returns -1 on failure and 0 on success, as it should.

(closes issue ASTERISK-17254)
Reported by: wybecom
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Merged revisions 373550 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373551 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 373552 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373553 65c4cc65-6c06-0410-ace0-fbb531ad65f3