net-test: packetdrill: Bugfix: Don't crash on FreeBSD when using an undefined symbol in a script.

This patch adds the missing end of table marker in the platform_symbols_table
for FreeBSD.

Signed-off-by: Michael Tuexen <tuexen@fh-muenster.de>

gtests/net/packetdrill/symbols_freebsd.c

@@ -298,6 +298,8 @@ struct int_symbol platform_symbols_table[] = {
 	{ ENOTCAPABLE,                      "ENOTCAPABLE"                     },
 	{ ECAPMODE,                         "ECAPMODE"                        },
 
+	/* Sentinel marking the end of the table. */
+	{ 0, NULL },
 };
 
 struct int_symbol *platform_symbols(void)