gh-152275: Initialize the optional attr argument to zero

The attr converter had no C default, so for methods where attr is an
optional argument (addch(), addstr(), insch(), ...) the generated code
left the attr_t variable uninitialized when the argument was omitted.
A garbage value was then applied as character attributes, corrupting the
output on platforms where the stack did not happen to be zero.  Give the
converter a default of 0, as the previous long converter had.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Modules/_cursesmodule.c

@@ -931,8 +931,9 @@ attr_converter(PyObject *arg, void *ptr)
 class attr_converter(CConverter):
     type = 'attr_t'
     converter = 'attr_converter'
+    c_default = '0'
 [python start generated code]*/
-/*[python end generated code: output=da39a3ee5e6b4b0d input=6132d3d99d3ec25a]*/
+/*[python end generated code: output=da39a3ee5e6b4b0d input=1e97aef32b7ed469]*/
 
 #ifdef HAVE_NCURSESW
 /* -------------------------------------------------------*/