[ES] Fixed potential error with loop

Signed-óff-by: Jim Martens <github@2martens.de>

es/blatt3/uebung3-4/uebung3-4.ino

@@ -95,9 +95,11 @@ void loop() {
         while (readable) {
             currentChar = Serial.read();
             readable = (currentChar != -1);
+            if (readable) {
                 command[i] = currentChar;
                 i++;
             }
+        }
         command[i] = '\0';
 
         if (strcmp(command, "LED_on") == 0) {