bpo-40679: Fix _PyEval_EvalCode() crash if qualname is NULL (GH-20615)

If name is NULL, name is now set to co->co_name.
If qualname is NULL, qualname is now set to name.

qualname must not be NULL: it is used to build error messages.

Cleanup also the code: declare variables where they are initialized.
Rename "name" local variables to "varname" to avoid overriding "name"
parameter.