Stop field_offset from crashing due to missing vtables

Now, a pointer to NULL is cast to the type in question, avoiding the need to
call new() or delete() with potentially-misaligned types. Also,
virtual_identity::find has been tweaked to prevent it from crashing on NULL
vtable pointers.

This was suggested by Angavrilov.

library/DataDefs.cpp

@@ -255,6 +255,9 @@ virtual_identity *virtual_identity::get(virtual_ptr instance_ptr)
 
 virtual_identity *virtual_identity::find(void *vtable)
 {
+    if (!vtable)
+        return NULL;
+
     // Actually, a reader/writer lock would be sufficient,
     // since the table is only written once per class.
     tthread::lock_guard<tthread::mutex> lock(*known_mutex);

library/lua/memscan.lua

@@ -307,14 +307,12 @@ function field_ref(handle,...)
 end
 
 function field_offset(type,...)
-    local handle = df.new(type)
-    local _,haddr = df.sizeof(handle)
-    local _,addr = df.sizeof(field_ref(handle,...))
-    -- to aid in diagnosis of bad virtual dtors
-    io.stderr:write('memscan: deleting instance of '..tostring(type) .. '\n'):flush()
-    df.delete(handle)
-    io.stderr:write('successfully deleted\n'):flush()
-    return addr-haddr
+    local tmp = df.new('intptr_t')  -- pointer to nullptr
+    local _, haddr = df.sizeof(tmp)
+    local handle = df.reinterpret_cast(type, tmp)
+    local _, addr = df.sizeof(field_ref(handle,...))
+    df.delete(tmp)
+    return addr - haddr
 end
 
 function MemoryArea:object_by_field(addr,type,...)