Fix null pointer deref (crash) in new_user() when "client" object is invalid

When system.new_user() was called but the current "client" object is uninitialized (e.g. has NULL protocol, host or IP address fields because there is no active client, e.g. because is was called from a timed event with active user online) - this code would dereference a NULL pointer and crash the b0rad. Reported by Mortifis.

src/sbbs3/js_system.c

@@ -1701,9 +1701,12 @@ js_new_user(JSContext *cx, uintN argc, jsval *arglist)
 		}
 	}
 	if(client!=NULL) {
-		SAFECOPY(user.modem,client->protocol);
-		SAFECOPY(user.comp,client->host);
-		SAFECOPY(user.ipaddr,client->addr);
+		if(client->protocol != NULL)
+			SAFECOPY(user.modem,client->protocol);
+		if(client->host != NULL)
+			SAFECOPY(user.comp,client->host);
+		if(client->addr != NULL)
+			SAFECOPY(user.ipaddr,client->addr);
 	}
 
 	user.sex=' ';