[MAINT] Refax for CSndBuffer::readData to make the call form clearer

[ethouris]

This changes the form of CSndBuffer::readData version with offset (intended for retransmission).

The problem with the existing solution was that the CPacket object passed there was used instrumentally to return the information about the sequence number range and message number for dropping. This didn't make any sense because the rule was that if a message retransmission was requested, but the message was qualified as to be dropped instead, the CPacket object wasn't in use at all. But then, when returning -1 as per drop request, the CPacket object was reused to return the message number of the message to be dropped that was written into its MSGNO field, which normally contains the message number AND the flags, but this time it was instrumentally used to just return the message number itself only. Simultaneously the msglen was returned because it was the only way to define the sequence range, while the first sequence from the range was set to the packet.

There has remained only the rule that the CPacket object is first set the message sequence number, and passing CPacket to the call of readData brings in this sequence information. But in case when the call resulted in a drop request, for which there was also defined a specified constant, the drop information is returned exclusively in the dedicated structure, which should be the only information for constructing the UMSG_DROPREQ message contents - the CPacket object is not in use in this case at all, even though it shares some partial information (the beginning sequence number).

There should be no change in the behavior, just the code was made more logically clear.