reco.C crash if MPDroot built without FFTW support

It is possible to build MPDroot without FFTW support (and without FFTW support in ROOT). In such case running reco.C leads to the crash since in files MpdTpcDigitizerAZ.cxx and MpdTpcDigitizerAZlt.cxx in the function SignalShaping() is not handled situation when fft[0] set by the call fft[0] = TVirtualFFT::FFT() is set to nullptr. Probably in such situation whole calculation should be terminated with proper message (does ROOT provide some exit function?).

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