[arbel] Assert that mapping length is non-zero

An (impossible) mapping length of zero produces a negative bit shift,
which is technically undefined.

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

src/drivers/infiniband/arbel.c

 	assert ( ( va & ( ARBEL_PAGE_SIZE - 1 ) ) == 0 );
 	assert ( ( pa & ( ARBEL_PAGE_SIZE - 1 ) ) == 0 );
 	assert ( ( len & ( ARBEL_PAGE_SIZE - 1 ) ) == 0 );
+	assert ( len != 0 );
 
 	/* Calculate starting points */
 	start = pa;