Don't try to run the last command if it's not installed

https://github.com/webmin/webmin/discussions/2161

useradmin/linux-lib.pl

@@ -21,10 +21,15 @@ return &password_file($config{'gshadow_file'}) ? 2 : 0;
 sub open_last_command
 {
 my ($fh, $user, $max) = @_;
+if (&has_command("last")) {
 	my $quser = quotemeta($user);
 	$max = " -n $max" if ($max);
 	open($fh, "(last -F -w$max $quser || last -w$max $quser) |");
 	}
+else {
+	open($fh, "</dev/null");
+	}
+}
 
 # read_last_line(handle)
 # Parses a line of output from last into an array of