py/stream: seek: Consistently handle negative offset for SEEK_SET.

Per POSIX, this is EINVAL, so raises OSError(EINVAL).

py/stream.c

@@ -448,11 +448,16 @@ STATIC mp_obj_t stream_seek(size_t n_args, const mp_obj_t *args) {
     struct mp_stream_seek_t seek_s;
     // TODO: Could be uint64
     seek_s.offset = mp_obj_get_int(args[1]);
-    seek_s.whence = 0;
+    seek_s.whence = SEEK_SET;
     if (n_args == 3) {
         seek_s.whence = mp_obj_get_int(args[2]);
     }
 
+    // In POSIX, it's error to seek before end of stream, we enforce it here.
+    if (seek_s.whence == SEEK_SET && seek_s.offset < 0) {
+        mp_raise_OSError(MP_EINVAL);
+    }
+
     int error;
     mp_uint_t res = stream_p->ioctl(args[0], MP_STREAM_SEEK, (mp_uint_t)(uintptr_t)&seek_s, &error);
     if (res == MP_STREAM_ERROR) {

tests/io/bytesio_ext2.py

@@ -0,0 +1,13 @@
+try:
+    import uio as io
+except ImportError:
+    import io
+
+a = io.BytesIO(b"foobar")
+try:
+    a.seek(-10)
+except Exception as e:
+    # CPython throws ValueError, but MicroPython has consistent stream
+    # interface, so BytesIO raises the same error as a real file, which
+    # is OSError(EINVAL).
+    print(repr(e))

tests/io/bytesio_ext2.py.exp

@@ -0,0 +1 @@
+OSError(22,)