librpcsec_gss: Fix an off-by-one in rpc_gss_get_principal_name()

Include an extra byte for the nul-terminator, otherwise we may end up
with an out-of-bounds write.

The corresponding bug in the kernel implementation was fixed by commit
e3081f7e3e ("kgssapi(4): Fix string overrun in Kerberos principal construction").

Reviewed by:	markj
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D57738

lib/librpcsec_gss/svc_rpcsec_gss.c

@@ -247,7 +247,7 @@ rpc_gss_get_principal_name(rpc_gss_principal_t *principal,
 	 * Construct a gss_buffer containing the full name formatted
 	 * as "name/node@domain" where node and domain are optional.
 	 */
-	namelen = strlen(name);
+	namelen = strlen(name) + 1;
 	if (node) {
 		namelen += strlen(node) + 1;
 	}