Fix refcount leak in error cases. Bug found by coverity.

Modules/_ctypes/cfield.c

@@ -1452,11 +1452,14 @@ Z_set(void *ptr, PyObject *value, Py_ssize_t size)
 		size += 1; /* terminating NUL */
 		size *= sizeof(wchar_t);
 		buffer = (wchar_t *)PyMem_Malloc(size);
-		if (!buffer)
+		if (!buffer) {
+			Py_DECREF(value);
 			return PyErr_NoMemory();
+		}
 		memset(buffer, 0, size);
 		keep = PyCObject_FromVoidPtr(buffer, PyMem_Free);
 		if (!keep) {
+			Py_DECREF(value);
 			PyMem_Free(buffer);
 			return NULL;
 		}