AP_HAL_Linux: UARTDriver: fix writting/reading with failures

When the buffer wraps and we do it in 2 steps, we can't actually do the
second part if it fails or if we wrote less bytes than we intended,
otherwise we will corrupt the data being sent.

libraries/AP_HAL_Linux/UARTDriver.cpp

@@ -429,8 +429,15 @@ bool UARTDriver::_write_pending_bytes(void)
             const auto n_vec = _writebuf.peekiovec(vec, n);
             for (int i = 0; i < n_vec; i++) {
                 ret = _write_fd(vec[i].data, (uint16_t)vec[i].len);
-                if (ret > 0)
-                    _writebuf.advance(ret);
+                if (ret < 0) {
+                    break;
+                }
+                _writebuf.advance(ret);
+
+                /* We wrote less than we asked for, stop */
+                if ((unsigned)ret != vec[i].len) {
+                    break;
+                }
             }
         }
     }
@@ -461,11 +468,15 @@ void UARTDriver::_timer_tick(void)
     const auto n_vec = _readbuf.reserve(vec, _readbuf.space());
     for (int i = 0; i < n_vec; i++) {
         ret = _read_fd(vec[i].data, vec[i].len);
-        if (ret < 0)
+        if (ret < 0) {
             break;
+        }
         _readbuf.commit((unsigned)ret);
-        if ((unsigned)ret < vec[i].len)
+
+        /* stop reading as we read less than we asked for */
+        if ((unsigned)ret < vec[i].len) {
             break;
+        }
     }
 
     _in_timer = false;