Fix Python crash on getprop deallocation

Fatal Python error: none_dealloc: deallocating None
Python runtime state: finalizing (tstate=0x000055c9bac70920)

Current thread 0x00007fbe34e47740 (most recent call first):
  <no Python frame>
Aborted (core dumped)

This is caused by a missing Py_INCREF on the returned Py_None, as
demonstrated e.g. in https://github.com/mythosil/swig-python-incref or
described at https://edcjones.tripod.com/refcount.html ("Remember to
INCREF Py_None!")

A PoC for triggering this crash is uploaded to
https://github.com/z3ntu/pylibfdt-crash .
With this patch applied to pylibfdt the crash does not happen.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Message-Id: <20211224102811.70695-1-luca@z3ntu.xyz>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

pylibfdt/libfdt.i

@@ -1040,14 +1040,16 @@ typedef uint32_t fdt32_t;
 
 /* typemap used for fdt_getprop() */
 %typemap(out) (const void *) {
-	if (!$1)
+	if (!$1) {
 		$result = Py_None;
-	else
+		Py_INCREF($result);
+	} else {
         %#if PY_VERSION_HEX >= 0x03000000
             $result = Py_BuildValue("y#", $1, (Py_ssize_t)*arg4);
         %#else
             $result = Py_BuildValue("s#", $1, (Py_ssize_t)*arg4);
         %#endif
+    }
 }
 
 /* typemap used for fdt_setprop() */