cc3200: Make telnet server ignore NULL characters.

This fixes paste mode (Ctrl-E) which was not working for the
telnet REPL.

cc3200/telnet/telnet.c

@@ -451,9 +451,13 @@ static void telnet_parse_input (uint8_t *str, int16_t *len) {
                 (*len)--;
                 _str++;
             }
-            else {
+            else if (*_str > 0) {
                 *str++ = *_str++;
             }
+            else {
+                _str++;
+                *len -= 1;
+            }
         }
         else {
             // in case we have received an incomplete telnet option, unlikely, but possible