Harden operator creation and reshape against null dereference and overflow - #11484

Summary

This PR hardens operator creation and reshape routines across multiple
operators in src/operators/ against null pointer dereferences, integer
overflows in stride, workspace, and reduction calculations, and invalid or
unbounded parameters:

1. constant-pad-nd:

   • Validate constant_pad_op_out and padding_value in
     xnn_create_constant_pad_nd_x8/x16/x32 before dereferencing.
   • Validate input_shape, pre_paddings, and post_paddings in
     reshape_constant_pad_nd when num_dims != 0.

2. slice-nd:

   • Validate slice_op_out in create_slice_nd.
   • Validate input_shape, offsets, and sizes in reshape_slice_nd.

3. transpose-nd:

   • Validate transpose_op_out in create_transpose_nd.
   • Validate perm and input_shape in reshape_transpose_nd.
   • Guard input_stride and output_stride multiplications and accumulation
     against integer overflow using xnn_safe_mul.

4. reduce-nd:

   • Validate reduce_op_out, params, and cvt_params in create_reduce_nd.
   • Validate num_input_dims, input_shape, num_reduction_axes, and
     reduction_axes in reshape_reduce_nd.
   • Bounds-check negative reduction axes against [-num_input_dims, 0).
   • Guard num_output_elements, workspace_size, num_reduction_elements,
     and zero_size against integer overflow using xnn_safe_mul and
     xnn_safe_add.

5. Pooling Operators:

   • Validate operator output pointers in argmax-pooling-nhwc,
     average-pooling-nhwc, max-pooling-nhwc, and unpooling-nhwc.
   • Validate params pointer when params_size != 0 in max-pooling-nhwc.

6. Unit Tests:

   • Added regression test cases covering null operator output pointers,
     null argument pointers, invalid axes, and stride/workspace overflows
     across test/operators/*-test.cc.

Testing

• Built and verified tests in clean build (build/): 53/53 tests passed.
• Built and verified with UBSan (build-ubsan/): 53/53 tests passed with zero
  sanitizer warnings.
• Verified all static and subgraph integration tests passed cleanly.
• Strict 80-character line length limit maintained throughout.