Skip lines that are greater than 67767976233561595

Sometimes this happens if keystroke-timestamps is killed without
flushing, such as by SIGKILL or loss of power.  When this happens,
it can write a partial line.  When the next run starts up, it
appends a timestamp to the partial line, resulting in a timestamp
too large for awk to process.

GNU awk's end-of-time behavior:

$ f() { awk '{ print strftime("%F %T", $1) }' <<< "$1"; }

$ f 67767976233561595
2147483647-12-31 23:59:52
$ f 67767976233561596
-2147483648-01-01 00:00:00

$ f 67768036191705595
-2147481749-12-31 23:59:52
$ f 67768036191705596
awk: cmd. line:1: (FILENAME=- FNR=1) fatal error: internal error
Aborted