STREscape: don't trim prematurely

STREscape: don't trim prematurely

STRescape holds strings in escape sequences such as OSC and DCS, and
its buffer is 512 bytes.

If the input is too big then trailing chars are ignored, but the test
was off-by-1 such that it took 510 chars instead of 511 (before a
terminating NULL is added).

Now the full size can be utilized.