[mypyc] Don't explicitly assign NULL values in setup functions (#15379)

While investigating something unrelated I stumbled across the `*_setup`
functions, and I noticed that they contain a lot of code that looks like
this:

```c
self->abc = NULL;
self->xyz = NULL;
// ...
```

Something told me that assigning `NULL` to a bunch of fields is propably
not needed, and when looking at the docs for `tp_alloc()` I found this
reference to
[`allocfunc`](https://docs.python.org/3/c-api/typeobj.html#c.allocfunc),
the typedef for `tp_alloc()`:

> It should return a pointer to a block of memory of adequate length for
the instance, suitably aligned, ***and initialized to zeros***, ...

Emphasis is mine.

Basically I added a simple check that removes lines that assigns `NULL`
values in setup functions. This removes about ~4100 lines from the C
file when self-compiling.

Author: dosisod

mypyc/codegen/emitclass.py

@@ -578,7 +578,12 @@ def generate_setup_for_class(
 
     for base in reversed(cl.base_mro):
         for attr, rtype in base.attributes.items():
-            emitter.emit_line(rf"self->{emitter.attr(attr)} = {emitter.c_undefined_value(rtype)};")
+            value = emitter.c_undefined_value(rtype)
+
+            # We don't need to set this field to NULL since tp_alloc() already
+            # zero-initializes `self`.
+            if value != "NULL":
+                emitter.emit_line(rf"self->{emitter.attr(attr)} = {value};")
 
     # Initialize attributes to default values, if necessary
     if defaults_fn is not None: