Prevent unitialized memory access in `GraphConstructor::MakeEdge`

The `MakeEdge` implementation assumes that there exists an output at `output_index` of `src` node and an input at `input_index` of `dst` node. However, if this is not the case this results in accessing data out of bounds. Because we are accessing an array that is a private member of a class and only in read only mode, this usually results only in unitialized memory access. However, it is reasonable to think that malicious users could manipulate these indexes to actually read data outside the class, thus resulting in information leakage and further exploits.

PiperOrigin-RevId: 346343288
Change-Id: I2127da27c2023d27f26efd39afa6c853385cab6f