gh-129813: Document that PyBytesWriter_GetData() cannot fail (#145900)

Document that PyBytesWriter_GetData() and PyBytesWriter_GetSize()
getter functions cannot fail

Author: vstinner

Doc/c-api/bytes.rst

@@ -371,13 +371,17 @@ Getters
 
    Get the writer size.
 
+   The function cannot fail.
+
 .. c:function:: void* PyBytesWriter_GetData(PyBytesWriter *writer)
 
    Get the writer data: start of the internal buffer.
 
    The pointer is valid until :c:func:`PyBytesWriter_Finish` or
    :c:func:`PyBytesWriter_Discard` is called on *writer*.
 
+   The function cannot fail.
+
 
 Low-level API
 ^^^^^^^^^^^^^

Modules/binascii.c

@@ -923,9 +923,6 @@ binascii_a2b_ascii85_impl(PyObject *module, Py_buffer *data, int foldspaces,
         return NULL;
     }
     unsigned char *bin_data = PyBytesWriter_GetData(writer);
-    if (bin_data == NULL) {
-        goto error;
-    }
 
     uint32_t leftchar = 0;
     int group_pos = 0;