* src/dispnew.c (sit_for): Return nil when interrupted by process output

* src/dispnew.c (sit_for): Return nil when interrupted by process output

Before adbb4eacc2a984c0fc0b65ec761368fd9067d6c5,
`read_and_dispose_of_process_output` called
`record_asynch_buffer_change` which added "artificial" input events
(in the form of BUFFER_SWITCH_EVENTs), causing sit_for to return
Qnil when interrupted by process output.  Without those BUFFER_SWITCH_EVENTs,
sit_for now tends to return Qt when interrupted by process output
making `read_char` believe that we've waited the whole timeout,
As consequence incoming process output tended to cause premature
auto-saving of files (sometimes right after almost every key press).

This patch recovers the previous behavior, which is not ideal
(incoming process output can delay auto-save indefinitely), but has
been good enough for many years.