Tidy USBSerial::attach() methods

Martino previously updated the attach() methods on the USBSerial object
to handle more than one callback to be called when data is received. I
updated that code to:
* Check that the _rx callback array doesn't overflow.
* Prepended a "_" prefix to the _rx and _howManyCallbacks members to
  make them consistent with the other members in this class and its
  parent USBCDC class.

Author: adamgreen

cores/arduino/USB/PluggableUSBSerial.h

@@ -157,8 +157,8 @@ class USBSerial: public USBCDC, public ::mbed::Stream, public HardwareSerial {
     {
         USBCDC::lock();
 
-        if ((mptr != NULL) && (tptr != NULL)) {
-            rx[howManyCallbacks++] = ::mbed::Callback<void()>(mptr, tptr);
+        if ((mptr != NULL) && (tptr != NULL) && (_howManyCallbacks < sizeof(_rx)/sizeof(_rx[0]))) {
+            _rx[_howManyCallbacks++] = ::mbed::Callback<void()>(mptr, tptr);
         }
 
         USBCDC::unlock();
@@ -173,8 +173,8 @@ class USBSerial: public USBCDC, public ::mbed::Stream, public HardwareSerial {
     {
         USBCDC::lock();
 
-        if (fptr != NULL) {
-            rx[howManyCallbacks++] = ::mbed::Callback<void()>(fptr);
+        if ((fptr != NULL) && (_howManyCallbacks < sizeof(_rx)/sizeof(_rx[0]))) {
+            _rx[_howManyCallbacks++] = ::mbed::Callback<void()>(fptr);
         }
 
         USBCDC::unlock();
@@ -189,7 +189,9 @@ class USBSerial: public USBCDC, public ::mbed::Stream, public HardwareSerial {
     {
         USBCDC::lock();
 
-        rx[howManyCallbacks++] = cb;
+        if (_howManyCallbacks < sizeof(_rx)/sizeof(_rx[0])) {
+            _rx[_howManyCallbacks++] = cb;
+        }
 
         USBCDC::unlock();
     }
@@ -273,9 +275,9 @@ class USBSerial: public USBCDC, public ::mbed::Stream, public HardwareSerial {
     }
 
 private:
-    ::mbed::Callback<void()> rx[MAX_CALLBACKS_ON_IRQ];
-    int howManyCallbacks = 0;
     void (*_settings_changed_callback)(int baud, int bits, int parity, int stop);
+    ::mbed::Callback<void()> _rx[MAX_CALLBACKS_ON_IRQ];
+    size_t _howManyCallbacks = 0;
 };
 }
 

cores/arduino/USB/USBSerial.cpp

@@ -95,14 +95,12 @@ void USBSerial::data_rx()
     USBCDC::assert_locked();
 
     //call a potential handler
-    int i = 0;
-    while (i < howManyCallbacks) {
-        if (rx[i]) {
-            rx[i].call();
+    for (size_t i = 0 ; i < _howManyCallbacks ; i++) {
+        if (_rx[i]) {
+            _rx[i].call();
         } else {
             break;
         }
-        i++;
     }
 }