[2.7] bpo-36289: Fix a possible reference leak in the io module (GH-12329)

Fix a reference leak in _bufferedreader_read_all():
_io.BufferedIOMixin.read() leaks a reference on 'data'
when it reads the whole file content but flush() fails.

Misc/NEWS.d/next/Library/2019-03-14-15-42-48.bpo-36289.wYKS47.rst

@@ -0,0 +1 @@
+Fix a possible reference leak in the io module.

Modules/_io/bufferedio.c

@@ -1363,6 +1363,7 @@ _bufferedreader_read_all(buffered *self)
         res = buffered_flush_and_rewind_unlocked(self);
         if (res == NULL) {
             Py_DECREF(chunks);
+            Py_XDECREF(data);
             return NULL;
         }
         Py_CLEAR(res);