On Windows, when a blinking cursor is enabled, PuTTY uses the system
default blink time from GetCaretBlinkTime(), which can be configured in
Control Panel.
Control Panel allows caret blinking to be disabled entirely, in which
case GetCaretBlinkTime() returns INFINITE. PuTTY wasn't handling this
case; if cursor blinking was enabled in PuTTY but disabled at the system
level, the terminal window would hang, blinking the cursor madly.