Bug: query caused new engine to panic

[Ben Ye]

-floor(histogram_quantile(-time(), tan(group without () ({series="2"} offset 4m14s))))
    engine_test.go:78: engine_test.go:78: ""
        
         unexpected error: unexpected error: runtime error: invalid memory address or nil pointer dereference

Similar to #200, this instant query could caused the new engine to panic during execution. Exec returns runtime error: invalid memory address or nil pointer dereference error while the old engine worked without issues on the same query.

[Michael Hoffmann]

Should also be fixed by #198 I think

[Ben Ye]

Right, seems the same issue.