Replace dead code with an assertion in winreg.c. (GH-10028)

diff --git a/PC/winreg.c b/PC/winreg.c
index 4f5a676..c6ad7ab 100644
--- a/PC/winreg.c
+++ b/PC/winreg.c
@@ -651,8 +651,7 @@
 
                     t = PyList_GET_ITEM(value, j);
                     wstr = PyUnicode_AsUnicodeAndSize(t, &len);
-                    if (wstr == NULL)
-                        return FALSE;
+                    assert(wstr);
                     wcscpy(P, wstr);
                     P += (len + 1);
                 }