C version of FFTPACK segfaults

OpalRingTest and RingCyclotron-Tests segfault as [found by Christoph] (ee8a32ba (comment 3812)) with:

[pc12290:01336] Signal: Segmentation fault (11)
[pc12290:01336] Signal code:  (128)
[pc12290:01336] Failing at address: (nil)
[pc12290:01336] [ 0] /lib64/libpthread.so.0[0x3ab660f7e0]
[pc12290:01336] [ 1] opal(rffti1_+0x1c)[0x25b43ac]
[pc12290:01336] [ 2] opal(rffti_+0x24)[0x25b4374]
[pc12290:01336] [ 3] opal(_ZN3FFTI11RCTransformLj3EdEC1ERK7NDIndexILj3EES5_RKbi+0x2c7)[0x13109d7]
[pc12290:01336] [ 4] opal(_ZN16FFTPoissonSolverC1EP16UniformCartesianILj3EdEP19CenteredFieldLayoutILj3ES1_4CellENSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESC_+0xf5c)[0x130428c]
[pc12290:01336] [ 5] opal(_ZN11FieldSolver10initSolverEP13PartBunchBaseIdLj3EE+0x7c5)[0x1038d35]
[pc12290:01336] [ 6] opal(_ZN8TrackRun16setupFieldsolverEv+0x1ef)[0x13efb6f]
[pc12290:01336] [ 7] opal(_ZN8TrackRun21setupCyclotronTrackerEv+0xbb)[0x13f3c2b]
[pc12290:01336] [ 8] opal(_ZN8TrackRun7executeEv+0x632)[0x13f5d02]
[pc12290:01336] [ 9] opal(_ZNK10OpalParser7executeEP6ObjectRKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+0x35)[0xff7f35]