set_pts memory leak

[janden]

I want to point out though that using cufinufft that way presupposes the user is carefully managing memory anyway, as CUFINUFFT_SETPTS always mallocs (which is assumed to be freed with the plan in CUFINUFFT_DESTROY) => so by setting new points without first destroying+creating a plan will cause a memory leak (this might be a good discussion in a different issue @MelodyShih @ahbarnett).

Originally posted by @JBlaschke in #71 (comment)

[MelodyShih]

Hi Joakim, I agree with the issue. I think adding a check to see if the arrays have already be allocated or not in function ALLOCGPUMEM2,3D_NUPTS before malloc them will fix it -- I will create a pull request.

[janden]

Fixed by #80.