fix #9037: segfault caused by incorrect alignment assumptions

The rand!(::MersenneTwister, A::Array{Float64}) assumed that the
array A was 16-byte aligned, which can be false (e.g. after a call
to resize!) and lead to a segfault in libdSFMT.
Merge pull request #9040 from JuliaLang/rf/fix-align

fix #9037: segfault in libdSFMT caused by unaligned Array