[dhcp] Copy exactly the required length when resizing DHCP options

When resizing DHCP options, iPXE currently calculates the length to be
copied by subtracting the destination pointer from the end of buffer
pointer.  This works and guarantees not to write beyond the end of the
buffer, but may end up reading beyond the end of the buffer.

Fix by calculating the required length exactly.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/dhcpopts.c

 	size_t new_encapsulator_len;
 	void *source;
 	void *dest;
-	void *end;
 	int rc;
 
 	/* Check for sufficient space */
 	option = dhcp_option ( options, offset );
 	source = ( ( ( void * ) option ) + old_len );
 	dest = ( ( ( void * ) option ) + new_len );
-	end = ( options->data + options->alloc_len );
-	memmove ( dest, source, ( end - dest ) );
+	memmove ( dest, source, ( new_used_len - offset - new_len ) );
 
 	/* Shrink options block, if applicable */
 	if ( new_used_len < options->alloc_len ) {