Investigate random fuzzing failures

[GiedriusS]

Run make fuzz locally and observe failures such as:

--- FAIL: FuzzEnginePromQLSmithInstantQuery (27.14s)
    --- FAIL: FuzzEnginePromQLSmithInstantQuery (0.46s)
        enginefuzz_test.go:450: load 30s
                                http_requests_total{pod="nginx-1", route="/"} 120.00+11.00x40
                                http_requests_total{pod="nginx-2", route="/"} -35+2.00x40
        enginefuzz_test.go:452: sum by (pod, route) (
              count(
                sinh(
                  asin(
                    (
                        avg({__name__="http_requests_total"} @ start())
                      and
                        absent(sort({__name__="http_requests_total",route="/"} offset 2m31s))
                    )
                  )
                )
              )
            )
        enginefuzz_test.go:454: case 21 error mismatch.
            new result: 
            old result: {} => 1 @[7000]
        enginefuzz_test.go:459: failed 1 test cases

Or

--- FAIL: FuzzEnginePromQLSmithInstantQuery (0.43s)
    --- FAIL: FuzzEnginePromQLSmithInstantQuery (0.21s)
        enginefuzz_test.go:450: load 30s
                                http_requests_total{pod="nginx-1", route="/"} -46.00+1.00x40
                                http_requests_total{pod="nginx-2", route="/"}  9+2.00x40
        enginefuzz_test.go:452: count_values without (__name__) ("value", sgn(-ln(idelta({__name__="http_requests_total"}[3m]))))
        enginefuzz_test.go:454: case 53 error mismatch.
            new result: {pod="nginx-1", route="/", value="0"} => 1 @[55000]
            {pod="nginx-2", route="/", value="-1"} => 1 @[55000]
            old result: {pod="nginx-1", route="/", value="-0"} => 1 @[55000]
            {pod="nginx-2", route="/", value="-1"} => 1 @[55000]
        enginefuzz_test.go:459: failed 1 test cases