16-byte align frames on the VMFrameStack

- Fixed: Don't assume operator new will return a pointer with 16-byte
  alignment when allocating a block for the VMFrameStack. Because it seems
  it's actually guaranteed to be 8-byte aligned. Don't know where I got
  the idea it would always be 16-byte aligned.

src/zscript/vm.h

@@ -682,6 +682,7 @@ struct VMFrame
 
 	VMValue *GetParam() const
 	{
+		assert(((size_t)this & 15) == 0 && "VM frame is unaligned");
 		return (VMValue *)(((size_t)(this + 1) + 15) & ~15);
 	}
 
@@ -789,6 +790,11 @@ private:
 		VMFrame *LastFrame;
 		VM_UBYTE *FreeSpace;
 		int BlockSize;
+
+		void InitFreeSpace()
+		{
+			FreeSpace = (VM_UBYTE *)(((size_t)(this + 1) + 15) & ~15);
+		}
 	};
 	BlockHeader *Blocks;
 	BlockHeader *UnusedBlocks;

src/zscript/vmframe.cpp

@@ -286,7 +286,7 @@ VMFrame *VMFrameStack::Alloc(int size)
 			block = (BlockHeader *)new VM_UBYTE[blocksize];
 			block->BlockSize = blocksize;
 		}
-		block->FreeSpace = (VM_UBYTE *)block + ((sizeof(BlockHeader) + 15) & ~15);
+		block->InitFreeSpace();
 		block->LastFrame = NULL;
 		block->NextBlock = Blocks;
 		Blocks = block;
@@ -340,7 +340,7 @@ VMFrame *VMFrameStack::PopFrame()
 		{
 			assert(Blocks->NextBlock == NULL);
 			Blocks->LastFrame = NULL;
-			Blocks->FreeSpace = (VM_UBYTE *)Blocks + ((sizeof(BlockHeader) + 15) & ~15);
+			Blocks->InitFreeSpace();
 		}
 		return NULL;
 	}