Generator should disallow input parameters with size mechanism: 'passed-in',

[marcoskirsch]

In PR #1104 it was noted that the generator could create slightly non-sensical code whenever an input parameter has its size as "mechanism: passed-in" (see comment).

That's because "passed-in" only makes sense for output parameters. We should add a check with a clear error message to the code-generator.

@texasaggie97 suggests

You could change codegen_helper.py line 331 to:
assert p['size']['mechanism'] != 'fixed-size' and p['size']['mechanism'] != 'passed-in'