Heap protect (#747)

Setting configENABLE_HEAP_PROTECTOR to 1 obfuscates heap
block pointers by XORing them with an application supplied
canary value. This obfuscation helps to catch heap corruption
should a heap buffer overflow occur.

This PR also adds heap bounds checking to heap_4 and heap_5.

This PR also adds some additional integer underflow checks.

include/FreeRTOS.h

@@ -1071,6 +1071,10 @@
     #define configAPPLICATION_ALLOCATED_HEAP    0
 #endif
 
+#ifndef configENABLE_HEAP_PROTECTOR
+    #define configENABLE_HEAP_PROTECTOR    0
+#endif
+
 #ifndef configUSE_TASK_NOTIFICATIONS
     #define configUSE_TASK_NOTIFICATIONS    1
 #endif