Fix TFTP reading of zero-size files:

Fix TFTP reading of zero-size files:

The AllocatePages EFI call returns an error when the allocation
size is 0.  Skip allocating memory and actually transferring the
file when it is empty.

Properly return the number of unread bytes so that a read() of n
bytes does not return n if no bytes were read.

While here, disallow lseek() beyond the TFTP file buffer for SEEK_CUR
as we already do for SEEK_SET.

ok patrick@