extmod/modselect: Handle growing the pollfds allocation correctly.

The poll_obj_t instances have their pollfd field point into this
allocation.  So if re-allocating results in a move, we need to update the
existing poll_obj_t's.

Update the test to cover this case.

Fixes issue #12887.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

tests/extmod/select_poll_fd.py

@@ -34,11 +34,22 @@ poller.register(1, select.POLLIN)
 # Poll for input, should return an empty list.
 print(poller.poll(0))
 
-# Test registering a very large number of file descriptors.
+# Test registering a very large number of file descriptors (will trigger
+# EINVAL due to more than OPEN_MAX fds).
 poller = select.poll()
 for fd in range(6000):
     poller.register(fd)
 try:
     poller.poll()
+    assert False
 except OSError as er:
     print(er.errno == errno.EINVAL)
+
+# Register stdout/stderr, plus many extra ones to trigger the fd vector
+# resizing. Then unregister the excess ones and verify poll still works.
+poller = select.poll()
+for fd in range(1, 1000):
+    poller.register(fd)
+for i in range(3, 1000):
+    poller.unregister(i)
+print(sorted(poller.poll()))