Add guards against uninitialized host pointer

An older fix (commit 72d0aa6) introduced the possiblity of dereferencing an
uninitialized host pointer by moving the getHostInst() call from USBHostMSD()'s
constructor to connect(). This is now solved by adding guards against an
uninitialized host pointer in checkResult(), SCSITransfer() and getMaxLun().
The practical implication of the bug was that mounting before connecting led
to a crash, but now just leads to an error message from the mount call.

Author: Alrik Vidstrom

src/USBHostMSD/USBHostMSD.cpp

@@ -187,6 +187,12 @@ int USBHostMSD::inquiry(uint8_t lun, uint8_t page_code)
 
 int USBHostMSD::checkResult(uint8_t res, USBEndpoint * ep)
 {
+    // Guard against host not being initialized
+    if (nullptr == host)
+    {
+        return -1;
+    }
+
     // if ep stalled: send clear feature
     if (res == USB_TYPE_STALL_ERROR) {
         res = host->controlWrite(   dev,
@@ -211,6 +217,12 @@ int USBHostMSD::SCSITransfer(uint8_t * cmd, uint8_t cmd_len, int flags, uint8_t
 
     int res = 0;
 
+    // Guard against host not being initialized
+    if (nullptr == host)
+    {
+        return -1;
+    }
+
     cbw.Signature = CBW_SIGNATURE;
     cbw.Tag = 0;
     cbw.DataLength = transfer_len;
@@ -310,6 +322,13 @@ int USBHostMSD::dataTransfer(uint8_t * buf, uint32_t block, uint8_t nbBlock, int
 int USBHostMSD::getMaxLun()
 {
     uint8_t buf[1], res;
+
+    // Guard against host not being initialized
+    if (nullptr == host)
+    {
+        return -1;
+    }
+
     res = host->controlRead(    dev, USB_RECIPIENT_INTERFACE | USB_DEVICE_TO_HOST | USB_REQUEST_TYPE_CLASS,
                                 0xfe, 0, msd_intf, buf, 1);
     USB_DBG("max lun: %d", buf[0]);

src/USBHostMSD/USBHostMSD.h

@@ -75,7 +75,7 @@ class USBHostMSD : public IUSBEnumerator, public mbed::BlockDevice
 
 
 private:
-    USBHost * host;
+    USBHost * host = nullptr;
     USBDeviceConnected * dev;
     bool dev_connected;
     USBEndpoint * bulk_in;