fixing and old unicode bug

the openos io buffer in utf8 mode can splice inside a utf8 sequence
this code prevents that by reading the next chunk to complete the sequence
in the case the stream actually has bad utf8 sequence, the io buffer decides to return
more data than it was asked, rather than corrupt the stream
closes #1207

src/main/resources/assets/opencomputers/loot/openos/lib/core/full_buffer.lua

@@ -142,20 +142,36 @@ function buffer:readBytesOrChars(readChunk, n)
     sub = unicode.sub
   end
   local data = ""
-  while len(data) ~= n do
-    if len(self.bufferRead) == 0 then
+  while true do
+    local current_data_len = len(data)
+    local needed = n - current_data_len
+    if needed < 1 then
+      break
+    end
+    -- if the buffer is empty OR there is only 1 char left, read next chunk
+    -- this is to protect that last byte from bad unicode
+    if #self.bufferRead == 0 then
       local result, reason = readChunk(self)
       if not result then
         if reason then
           return result, reason
         else -- eof
-          return #data > 0 and data or nil
+          return current_data_len > 0 and data or nil
         end
       end
     end
-    local left = n - len(data)
-    data = data .. sub(self.bufferRead, 1, left)
-    self.bufferRead = sub(self.bufferRead, left + 1)
+    local splice = self.bufferRead
+    if len(self.bufferRead) > needed then
+      splice = sub(self.bufferRead, 1, needed)
+      if len(splice) ~= needed then
+        -- this can happen if the stream does not represent valid utf8 sequences
+        -- we could search the string for the bad sequence but regardless, we're going to just return the raw data
+        splice = self.bufferRead -- yes this is more than the user is asking for, but this is better than corrupting the stream
+      end
+    -- else -- we will read more chunks
+    end
+    data = data .. splice
+    self.bufferRead = string.sub(self.bufferRead, #splice + 1)
   end
   return data
 end