Handle non-printable characters in vga buffer module (#425)

src/vga_buffer.rs

@@ -107,7 +107,13 @@ impl Writer {
     /// mode.
     fn write_string(&mut self, s: &str) {
         for byte in s.bytes() {
-          self.write_byte(byte)
+            match byte {
+                // printable ASCII byte or newline
+                0x20...0x7e | b'\n' => self.write_byte(byte),
+                // not part of printable ASCII range
+                _ => self.write_byte(0xfe),
+            }
+
         }
     }