test-compress-benchmark: fix argument parsing on 32bit

The patch is not minimal, but a function to parse size_t is probably
going to come in handy in other places, so I think it's nicer to define
a proper parsing function than to open-code the cast.

src/basic/copy.c

@@ -71,7 +71,7 @@ static ssize_t try_copy_file_range(int fd_in, loff_t *off_in,
 int copy_bytes(int fdf, int fdt, uint64_t max_bytes, bool try_reflink) {
         bool try_cfr = true, try_sendfile = true, try_splice = true;
         int r;
-        size_t m = SSIZE_MAX; /* that the maximum that sendfile and c_f_r accept */
+        size_t m = SSIZE_MAX; /* that is the maximum that sendfile and c_f_r accept */
 
         assert(fdf >= 0);
         assert(fdt >= 0);