Use terse check of bytes_left in sparseencode.c

...and pair with a more verbose description of what is being checked,
to clarify the meaning of "if (!(bytes_left -= bytes_read))", since
the terseness here is for illustrative purposes of using the evaluated
value of an assignment and the not operator to check for a zero numeric
value rather than an == comparison to 0 (either making sense in a case
like this, which could be read as "when there are 0 bytes left after
subtracting what we've read in" or "when there are not any bytes left
after subtracting what we've read in".