core/frame: generate correct T.140 payload in ast_sendtext_data()

ast_sendtext_data() would create an incorrect T.140 text frame which
length include the null terminator byte. It causes ultimately RTP
packets to be send with this trailing 0. The proposed fix just set the
correct length to the text frame

ASTERISK-28089
Reported by: Emmanuel BUU
Tested by: Emmanuel BUU

Change-Id: I7ab1b9ed1e21683b2b667ea0a59d9aba3c77dd96