Convert method for StaticArray shouldn't access data field

[andreasnoack]

#550 introduces

StaticArrays.jl/src/convert.jl

Line 6 in 1d5524c

@inline (::Type{SA})(a::StaticArray) where {SA<:SizedArray} = SA(a.data)

that accesses data for StaticArray but since the latter is an abstract type, there is no reason to believe that it has a data field. This is e.g. invalid for https://github.com/SciML/LabelledArrays.jl/blob/bd9cd38dc9f4ef4eb366c16492fa64ca795eb566/src/slarray.jl#L1-L19