Refactor record padding handling to eliminate middle padding pattern

Move padSz index advancement from individual message handlers to a
single location at the end of record processing in ProcessReply.
Previously, each handler (DoFinished, DoApplicationData, DoAlert,
DoCertificate, DoServerHello, etc.) advanced the index by padSz,
which then had to be corrected when processing multiple messages
in one record. This "middle padding" pattern was error-prone.

Now curSize is reduced by padSz after decryption/verification so
it reflects content size only. Message handlers advance the index
by content size only. padSz is added once when the record is fully
processed. The correction code for multi-message records is removed.

Author: julek-wolfssl