gmain: handle blocked source in g_source_add_child_source()

gmain: handle blocked source in g_source_add_child_source()

When a child_source is added to a blocked source it has no context, yet we
call block_source on it that segfaults when it dereferences the NULL context
when it attempts to remove the file descriptors. To fix this we:

- Ensure that when we block a source, we don't attempt to remove its file
  descriptors from a NULL context.

- Also ensure that when we attach a blocked source to a context, we don't add the
  file descriptors to the context.

https://bugzilla.gnome.org/show_bug.cgi?id=701283