VPP PAPI: Error in unserializer for non-array compound types.

A bug in the decoder of messages when there was a non-array compound type.
The typical result was an error message from the struct library:
 "error:unpack_from requires a buffer of at least 4 bytes"

Change-Id: Ie30fec6fc39b9f4177b54fa4adc4fc69674f0e12
Signed-off-by: Ole Troan <ot@cisco.com>
diff --git a/src/vpp-api/python/vpp_papi.py b/src/vpp-api/python/vpp_papi.py
index 7b66c0f..67ac22a 100644
--- a/src/vpp-api/python/vpp_papi.py
+++ b/src/vpp-api/python/vpp_papi.py
@@ -360,6 +360,7 @@
                         size += v[1].size
             else:
                 if callable(v):
+                    size = 0
                     (s, l) = v(self, False, buf, off, None)
                     res.append(l)
                     size += s