avoid segfaults due to double free

If fftw_mpi_mkplans_posttranspose() fails, the plans cld3, cld2rest,
and cld2 are destroyed at nada and must be set to NULL. Otherwise,
a second destroy at nada in mkplan() will cause a segfault.
Merge pull request #27 from mpip/master

avoid segfaults due to double free