Fix I2C slave issue: return correct size

Fix #212

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

Author: fpistm

cores/arduino/stm32/twi.c

@@ -539,7 +539,7 @@ void HAL_I2C_ListenCpltCallback(I2C_HandleTypeDef *hi2c)
 
   if((obj->i2c_onSlaveReceive != NULL) &&
     (obj->slaveMode == SLAVE_MODE_RECEIVE)) {
-    nbData = I2C_TXRX_BUFFER_SIZE - obj->handle.XferCount;
+    nbData = I2C_TXRX_BUFFER_SIZE - obj->handle.XferSize;
     if(nbData != 0) {
       obj->i2c_onSlaveReceive(obj->i2cTxRxBuffer, nbData);
     }

libraries/Wire/src/Wire.cpp

@@ -381,13 +381,12 @@ void TwoWire::onReceiveService(uint8_t* inBytes, int numBytes)
     return;
   }
 
+  allocateRxBuffer(numBytes);
+  // error if no memory block available to allocate the buffer
   if(rxBuffer == nullptr){
-    allocateRxBuffer(numBytes);
-    // error if no memory block available to allocate the buffer
-    if(rxBuffer == nullptr){
-      Error_Handler();
-    }
+    Error_Handler();
   }
+
   // copy twi rx buffer into local read buffer
   // this enables new reads to happen in parallel
   memcpy(rxBuffer, inBytes, numBytes);