fix segfault in nufft_precond_create
fix segfault in nufft_precond_create

pdata->N was used uninitialized in this function, leading to
unpleasantness

sdimoudi/bart,sdimoudi/bart,mrirecon/bart,mrirecon/bart,sidward/bart,sdimoudi/bart,mrirecon/bart,sidward/bart,sidward/bart,Takishima/bart,Takishima/bart,mrirecon/bart,mrirecon/bart,Takishima/bart,sdimoudi/bart,sidward/bart,Takishima/bart,sdimoudi/bart,Takishima/bart,sidward/bart