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

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

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@373550 65c4cc65-6c06-0410-ace0-fbb531ad65f3