[main] replace computed value by a constant

[main] replace computed value by a constant

The reason why mkbundle is not working is that for some reason
this constant expression is producing different code than the
-24 that it is supposed to.

What is odd is that the return value from the lseek system
call is the same, but yet, read fails with an end-of-file with
this value, and otherwise it passes.

With this call:

     lseek (fd, -(16+sizeof(uint64_t)), SEEK_END)

strace displays:

lseek(0x3, 0xFFFFFFE8, 0x2)		 = 19375512 0

With this call:

     lseek (fd, -24, SEEK_END)

strace displays:

lseek(0x3, 0xFFFFFFFFFFFFFFE8, 0x2)	 = 19375512 0

Now, while both results from lseek are the same, a subsequent read
with 24 bytes to read returns zero on the former case, and 24 on
the latter case.