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This cleans up a recent fix related to Bug#29661.
Suggested by Stefan Monnier in:
https://lists.gnu.org/r/emacs-devel/2017-12/msg00544.html
* src/frame.c (next_frame, prev_frame, delete_frame):
Restore debugging checks that Vframe_list is non-nil,
as FOR_EACH_FRAME no longer has these checks.
(delete_frame): Remove no-longer-needed checks that Vframe_list is
non-nil, as FOR_EACH_FRAME no longer assumes that.
* src/frame.h (FOR_EACH_FRAME): Do not assume Vframe_list is non-nil.