Don't Py_DECREF NULL variable in io.IncrementalNewlineDecoder.

Found with Clang's Static Analyzer.

Misc/NEWS

@@ -19,6 +19,8 @@ Core and Builtins
 Library
 -------
 
+- Don't Py_DECREF NULL variable in io.IncrementalNewlineDecoder.
+
 - Issue #8515: Set __file__ when run file in IDLE.
   Initial patch by Bruce Frederiksen.
 

Modules/_io/textio.c

@@ -460,7 +460,7 @@ _PyIncrementalNewlineDecoder_decode(PyObject *_self,
             output = PyUnicode_FromKindAndData(kind, translated, out);
             PyMem_Free(translated);
             if (!output)
-                goto error;
+                return NULL;
         }
         self->seennl |= seennl;
     }