libjail: Guard against programmer error in jailparam_export()

If the caller didn't use jailparam_import() to fetch the parameter
value, an attempt to export it will trigger a segfault.  Make it a bit
easier to figure out what's happening in this situation.

PR:		276809
Reviewed by:	jamie
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D43732

lib/libjail/jail.c

@@ -737,6 +737,12 @@ jailparam_export(struct jailparam *jp)
 	int i, nval, ival;
 	char valbuf[INET6_ADDRSTRLEN];
 
+	if (jp->jp_value == NULL) {
+		snprintf(jail_errmsg, JAIL_ERRMSGLEN,
+		    "parameter %s was not imported", jp->jp_name);
+		errno = EINVAL;
+		return (NULL);
+	}
 	if ((jp->jp_ctltype & CTLTYPE) == CTLTYPE_STRING) {
 		value = strdup(jp->jp_value);
 		if (value == NULL)