Correct a NULL pointer deference in nslookup and nsupdate that would

cause the utility to crash in interactive mode when the user gives
an EOF on standard input.

MFC after:	3 days
Approved by:	re (gjb)

contrib/bind9/bin/dig/nslookup.c

@@ -767,7 +767,8 @@ get_next_command(void) {
 	if (interactive) {
 #ifdef HAVE_READLINE
 		ptr = readline("> ");
-		add_history(ptr);
+		if (ptr != NULL && *ptr != '\0')
+			add_history(ptr);
 #else
 		fputs("> ", stderr);
 		fflush(stderr);

contrib/bind9/bin/nsupdate/nsupdate.c

@@ -2008,7 +2008,8 @@ get_next_command(void) {
 	if (interactive) {
 #ifdef HAVE_READLINE
 		cmdline = readline("> ");
-		add_history(cmdline);
+		if (cmdline != NULL && *cmdline != '\0')
+			add_history(cmdline);
 #else
 		fprintf(stdout, "> ");
 		fflush(stdout);