SAP: make the Server Name field FT_STRINGZPAD.

According to the Novell IPX Router Specification, Chapter 4 "Service
Advertising Protocol (SAP)":

	Server Name

	This field contains the 48 byte character string name that is
	assigned to a server.  The Server Name, in combination with the
	Service Type, uniquely identifies a server on an internetwork.
	Although SAP response packets always include the full 48 bytes
	for this field, typical server names are usually less than 48
	characters long and are ASCII NULL terminated.  The contents of
	the unused bytes which follow the NULL terminator are undefined.

which seems to indicate that a full 48-byte name will not have a null
termintor.  It also indicates that the field isn't null-padded, just
"null-terminated if it's not terminated by the end of the field's fixed
length"; perhaps we need to distinguish between the former and the
latter, although it's not clear what would be a good short name for the
latter.

In any case, it sounds as if it's not guaranteed to be null-terminated.

epan/dissectors/packet-ipx.c

@@ -1486,7 +1486,7 @@ proto_register_ipx(void)
 
 		{ &hf_sap_server_name,
 		{ "Server Name",		"ipxsap.server.name",
-		  FT_STRINGZ,	BASE_NONE,	NULL,	0x0,
+		  FT_STRINGZPAD,	BASE_NONE,	NULL,	0x0,
 		  NULL, HFILL }},
 
 		{ &hf_sap_server_network,