Docs for -D authorized_keys, check for empty

An empty -D doesn't make sense, so reject it.

manpages/dropbear.8

@@ -116,6 +116,10 @@ Disregard the command provided by the user and always run \fIforced_command\fR.
 overrides any authorized_keys command= option. The original command is saved in the 
 SSH_ORIGINAL_COMMAND environment variable (see below).
 .TP
+.B \-D \fIauthorized_keys_dir
+Specify the directory to use for authorized_keys files. The default is ~/.ssh , paths with
+a leading ~/ will be home directory expanded.
+.TP
 .B \-V
 Print the version
 

src/svr-runopts.c

@@ -456,6 +456,10 @@ void svr_getopts(int argc, char ** argv) {
 		dropbear_exit("-t and -s are incompatible");
 	}
 
+	if (strlen(svr_opts.authorized_keys_dir) == 0) {
+		dropbear_exit("Bad -D");
+	}
+
 #if DROPBEAR_PLUGIN
 	if (pubkey_plugin) {
 		svr_opts.pubkey_plugin = m_strdup(pubkey_plugin);