[patch] bounds checking in send_file_readX

[Dmitry Shatrov]

In send_file_readX(), if startpos > sbuf.st_size, then smb_maxcnt is set
to an invalid large value due to integer overflow.
As for me, this resulted in MS Word hanging while trying to save
a 1.5Mb document.

Introduced by the following patch: 
http://viewcvs.samba.org/cgi-bin/viewcvs.cgi/branches/SAMBA_3_0/source/smbd/reply.c?rev=22920&r1=22846&r2=22920

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