proc/escape.c: Prevent integer overflows in escape_str_utf8().

proc/escape.c: Prevent integer overflows in escape_str_utf8().

Simply rearrange the old comparisons. The new comparisons are safe,
because we know from previous checks that:

1/ wlen > 0

2/ my_cells < *maxcells (also: my_cells >= 0 and *maxcells > 0)

3/ len > 1

4/ my_bytes+1 < bufsize (also: my_bytes >= 0 and bufsize > 0)