Fix sparkfun#454: use-after-free in Wire and SPI libraries.

Also add the WireShim to properly shutdown the Wire
hardware.

Author: nigelb

libraries/SPI/src/SPI.cpp

@@ -86,6 +86,7 @@ void arduino::MbedSPI::begin() {
 void arduino::MbedSPI::end() {
     if (dev) {
         delete dev;
+        dev = NULL;
     }
 }
 

libraries/Wire/src/Wire.cpp

@@ -13,7 +13,7 @@ arduino::MbedI2C::MbedI2C(int sda, int scl) : _sda(sda), _scl(scl), usedTxBuffer
 
 void arduino::MbedI2C::begin() {
 	if(!master){
-		master = new mbed::I2C((PinName)_sda, (PinName)_scl);
+		master = new WireShim((PinName)_sda, (PinName)_scl);
 		setClock(100000); //Default to 100kHz
 	}
 }
@@ -31,6 +31,7 @@ void arduino::MbedI2C::begin(uint8_t slaveAddr) {
 void arduino::MbedI2C::end() {
 	if (master != NULL) {
 		delete master;
+		master = NULL;
 	}
 #ifdef DEVICE_I2CSLAVE
 	if (slave != NULL) {

libraries/Wire/src/Wire.h

@@ -19,6 +19,15 @@ typedef void (*voidFuncPtrParamInt)(int);
 
 namespace arduino {
 
+class WireShim: public mbed::I2C
+{
+    public:
+    using mbed::I2C::I2C;
+    virtual ~WireShim(){
+        i2c_free(&_i2c);
+    };
+};
+
 class MbedI2C : public HardwareI2C
 {
   public:
@@ -52,7 +61,7 @@ class MbedI2C : public HardwareI2C
 #ifdef DEVICE_I2CSLAVE
     mbed::I2CSlave* slave;
 #endif
-    mbed::I2C*      master;
+    WireShim*      master;
     int _sda;
     int _scl;
     int _address;