Update range check when reference is 0 or inf

The verification library's tosa_reference_check_fp_bnd function
forced implementations to yield exactly 0 if the reference model yields
0 and exactly inf if it yields inf.
Now we compute error bounds as normal and allow the same error range as
we would have allowed otherwise in those cases.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I065f7f0bda292d5d112614a4fd013f9a4694b9c9