Fix display of cursor when 'blink-cursor-delay' has small value

Fix display of cursor when 'blink-cursor-delay' has small value

* lisp/frame.el (blink-cursor-check, blink-cursor-mode):
Protect ourselves against too small values of blink-cursor-delay.
This avoids erratic display of the cursor, or even failure to
display it, when user types text at high speed or leans on a key
to invoke the keyboard auto-repeat feature.  (Bug#24372)