Fix various memory safety issues

- Fixed implicit type promotion in VkDeviceSize calculation by explicitly casting
  SMALL_BUFFER_SIZE and shift operations to prevent sign extension.

- Ensured null termination in VmaStringBuilder::Add by resizing the buffer and
  appending '\0' after copying.

- Initialized class member m_hParentPool in the VmaDeviceMemoryBlock constructor
  to prevent potential undefined behavior.

include/vk_mem_alloc.h

@@ -5651,8 +5651,9 @@ void VmaStringBuilder::Add(const char* pStr)
     if (strLen > 0)
     {
         const size_t oldCount = m_Data.size();
-        m_Data.resize(oldCount + strLen);
+        m_Data.resize(oldCount + strLen + 1);
         memcpy(m_Data.data() + oldCount, pStr, strLen);
+        m_Data[oldCount + strLen] = '\0';
     }
 }
 
@@ -9146,7 +9147,7 @@ bool VmaBlockMetadata_TLSF::CreateAllocationRequest(
 
     // Round up to the next block
     VkDeviceSize sizeForNextList = allocSize;
-    VkDeviceSize smallSizeStep = VkDeviceSize(SMALL_BUFFER_SIZE / (IsVirtual() ? 1 << SECOND_LEVEL_INDEX : 4));
+    VkDeviceSize smallSizeStep = static_cast<VkDeviceSize>(SMALL_BUFFER_SIZE) / static_cast<VkDeviceSize>(IsVirtual() ? static_cast<VkDeviceSize>(1) << SECOND_LEVEL_INDEX : 4);
     if (allocSize > SMALL_BUFFER_SIZE)
     {
         sizeForNextList += (1ULL << (VMA_BITSCAN_MSB(allocSize) - SECOND_LEVEL_INDEX));
@@ -10594,6 +10595,7 @@ static void vma_delete_array(VmaAllocator hAllocator, T* ptr, size_t count)
 #ifndef _VMA_DEVICE_MEMORY_BLOCK_FUNCTIONS
 VmaDeviceMemoryBlock::VmaDeviceMemoryBlock(VmaAllocator hAllocator)
     : m_pMetadata(VMA_NULL),
+    m_hParentPool(nullptr),
     m_MemoryTypeIndex(UINT32_MAX),
     m_Id(0),
     m_hMemory(VK_NULL_HANDLE),