Bug fixes: output param initialization, output var symbol address

Bug fix: was not properly initializing the default values of output params if they were written but never read. Most of the time, the output param was written over in the shader body anyway, so this was not noticed.  But in cases where it might not be assigned in the main shader body, the default value was not present.

Bug fix: Avoid incorrect heap addressing when retrieving a symbol's address.

Along the way, another minor improvement: Nicer debug printing when op doesn't have an associated source line.