#28052 Fix heap OOB read in RNN operator via sequence_lens=0

Description

In the CPU RNN operator's \Assign_Y_h\ function, when \sequence_lens\ contains a value of 0, the computation \sequence_lens[batch] - 1 = -1\ produces a negative offset into the Y output buffer. \CopyVector\ then reads \hidden_size\ floats from heap memory before the buffer, leaking heap data into the \Y_h\ output tensor.

LSTM and GRU already handle zero-length sequences correctly (early return + zero-fill in compute path), but the basic RNN operator had neither protection.

Changes

• rnn.cc \Compute()\: Add early return when \max_sequence_length == 0\ — zero-fills Y and Y_h outputs and returns immediately (matches existing LSTM/GRU pattern)
• rnn.cc \Assign_Y_h()\: Add bounds check on \last_time_step\ before computing buffer offset — guards against both negative index (\seq_lens=0\) and index >= seq_length, zero-fills Y_h for invalid entries