TMemoryBuffer: using lua string in wrong way

1.
local val = string.sub(self.buffer, self.rPos, len)
The call string.sub(s,i,j) extracts a piece of the string s,
from the i-th to the j-th character inclusive.
In Lua, the first character of a string has index 1.

2.
self.buffer = self.buffer + buf
self.wPos = self.wPos + buf
.. is the string concatenation operator in Lua, not +.