Don't Py_DECREF NULL variable in io.IncrementalNewlineDecoder.

Found with Clang's Static Analyzer.
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 833a527..ae105e5 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -460,7 +460,7 @@
             output = PyUnicode_FromKindAndData(kind, translated, out);
             PyMem_Free(translated);
             if (!output)
-                goto error;
+                return NULL;
         }
         self->seennl |= seennl;
     }