F4: USBD_StrDesc[] size fix

- bug: memory overflow for size values less than 62, in function USBD_GetString().
The array in question (USBD_StrDesc) is followed in memory by 2 fill bytes for the current compile order.
Arduino 1.8.6 compiles the files in different order, so that the USB will not working anymore.
The longest string is 32 bytes long (plus ending zero), and 2 more bytes will be added at the beginning (https://github.com/rogerclarkmelbourne/Arduino_STM32/blob/master/STM32F4/cores/maple/libmaple/usbF4/STM32_USB_Device_Library/Core/src/usbd_req.c#L818-L834).
The last 2 bytes are fill bytes (align 4).

Author: stevstrong

STM32F4/cores/maple/libmaple/usbF4/VCP/usbd_conf.h

@@ -32,7 +32,7 @@
   */
 #define USBD_CFG_MAX_NUM                1
 #define USBD_ITF_MAX_NUM                1
-#define USB_MAX_STR_DESC_SIZ            50
+#define USB_MAX_STR_DESC_SIZ            (64+4) // longest descriptor string length + 2
 
 /** @defgroup USB_VCP_Class_Layer_Parameter
   * @{