HAL_PX4: prevent read past end of buffer

libraries/AP_HAL_PX4/UARTDriver.cpp

@@ -503,7 +503,7 @@ void PX4UARTDriver::_timer_tick(void)
             // split into two writes
             uint16_t n1 = _writebuf_size - _writebuf_head;
             int ret = _write_fd(&_writebuf[_writebuf_head], n1);
-            if (ret == n1 && n != n1) {
+            if (ret == n1 && n > n1) {
                 _write_fd(&_writebuf[_writebuf_head], n - n1);                
             }
         }
@@ -523,7 +523,7 @@ void PX4UARTDriver::_timer_tick(void)
             uint16_t n1 = _readbuf_size - _readbuf_tail;
             assert(_readbuf_tail+n1 <= _readbuf_size);
             int ret = _read_fd(&_readbuf[_readbuf_tail], n1);
-            if (ret == n1 && n != n1) {
+            if (ret == n1 && n > n1) {
                 assert(_readbuf_tail+(n-n1) <= _readbuf_size);
                 _read_fd(&_readbuf[_readbuf_tail], n - n1);                
             }