Fix Full.ino test directory

It is possible to use only a File without open it but in this case
boolean test should not be used as the file is not opened.
Moreover, in this case the close should be called to freed dynamic
allocated name.

Fix #12

Signed-off-by: Frederic.Pillon <frederic.pillon@st.com>

Author: fpistm

examples/Full/Full.ino

@@ -157,13 +157,14 @@ void setup()
   }
 
   /* Test isDirectory() method */
-  MyFile = File("STM32");
+  MyFile = SD.open("STM32");
   if (MyFile) {
     Serial.print("Is 'STM32' is a dir: ");
     if (MyFile.isDirectory())
       Serial.println("OK");
     else
       Serial.println("KO");
+    MyFile.close();
   } else {
     Serial.println("Error to open 'STM32' dir");
   }