papi: use correct size for fixed length strings

This is required for correct packing of unions.

Type: fix
Change-Id: I92d04b1bbcb3fc3fe06474d8f2c59ed3502e6bd6
Signed-off-by: Klement Sekera <klement.sekera@gmail.com>

src/vpp-api/python/vpp_papi/vpp_serializer.py

@@ -135,7 +135,7 @@ class String(Packer):
     def __init__(self, name, num, options):
         self.name = name
         self.num = num
-        self.size = 1
+        self.size = num if num else 1
         self.length_field_packer = BaseTypes("u32")
         self.limit = options["limit"] if "limit" in options else num
         self.fixed = True if num else False