Fix #401, use valid size when calling CFE_MSG_Init

Use the offset of the payload start, which by definition must include at
least enough space for the primary header.  This ensures that the value
will pass any size check enforced in CFE_MSG_Init.

fsw/src/cf_cfdp_sbintf.c

@@ -118,7 +118,7 @@ CF_Logical_PduBuffer_t *CF_CFDP_MsgOutGet(const CF_Transaction_t *t, bool silent
         if (success)
         {
             CFE_MSG_Init(&CF_AppData.engine.out.msg->Msg,
-                         CFE_SB_ValueToMsgId(CF_AppData.config_table->chan[t->chan_num].mid_output), 0);
+                         CFE_SB_ValueToMsgId(CF_AppData.config_table->chan[t->chan_num].mid_output), offsetof(CF_PduTlmMsg_t, ph));
             ++CF_AppData.engine.outgoing_counter; /* even if max_outgoing_messages_per_wakeup is 0 (unlimited), it's ok
                                                     to inc this */