while (myFile.available()) {
myFile.read(buf,10);
if (myFile.read() == '\n') {
client.println(buf);
}
}
As long as there is data to read, read 10 characters, and store them in an array that can hold 10 characters. Then, pass that non-NULL terminated array to a function that expects a NULL terminated array, and you get garbage out.
Make your array one element larger. Capture the return value from File::read() (the number of bytes actually read), and use that value as an array index, and store a NULL at that location. No more garbage will be sent.