do_switch_frame: before leaving mini-window, check other (mru) window is live

do_switch_frame: before leaving mini-window, check other (mru) window is live

This fixes bug#55684.  There, with a minibuffer-only frame at start up,
Emacs tried to switch to this frame, whose selected window was the
mini-window.  There is no other active window in this frame, so the
attempt to swith to another window failed.

* src/frame.c (do_switch_frame): On switching to a frame whose selected
window is as above, before selecting the most recently used window, check
this ostensible window is an actual live window.  Otherwise leave the
mini-window selected.