HeapBlockDevice::erase(): free up heap memory

`HeapBlockDevice::erase()` previously performed a range and alignment
check only. This commit adds freeing of heap memory.

storage/blockdevice/include/blockdevice/HeapBlockDevice.h

@@ -28,9 +28,11 @@
 
 namespace mbed {
 
-/** Lazily allocated heap-backed block device
+/** Lazily allocated heap-backed block device.
  *
- * Useful for simulating a block device and tests
+ * Useful for simulating a block device and tests.
+ *
+ * @note Each block is allocated when used, and freed when erased.
  *
  * @code
  * #include "mbed.h"

storage/blockdevice/source/HeapBlockDevice.cpp

@@ -44,7 +44,7 @@ HeapBlockDevice::~HeapBlockDevice()
         }
 
         delete[] _blocks;
-        _blocks = 0;
+        _blocks = nullptr;
     }
 }
 
@@ -63,7 +63,7 @@ int HeapBlockDevice::init()
         }
 
         for (size_t i = 0; i < _count; i++) {
-            _blocks[i] = 0;
+            _blocks[i] = nullptr;
         }
     }
 
@@ -184,6 +184,13 @@ int HeapBlockDevice::erase(bd_addr_t addr, bd_size_t size)
     if (!is_valid_erase(addr, size)) {
         return BD_ERROR_DEVICE_ERROR;
     }
+
+    for (size_t i = 0; i < (size / _erase_size); i++) {
+        size_t index = addr / _erase_size + i;
+        delete[] _blocks[index];
+        _blocks[index] = nullptr;
+    }
+
     return 0;
 }