soft decoding: fix calc_soft_dec() crash dute to uninitialized pre_diff_...

The constellation soft decoder crashes with this test script, the patch fixes this by checking whether the pre_diff_code is initialized.

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#!/usr/bin/env python

from gnuradio import gr, blocks, digital, analog

con = digital.constellation_bpsk()
# con = digital.constellation_qpsk()

top = gr.top_block()
top.connect(analog.fastnoise_source_c(analog.GR_GAUSSIAN, 1.0),
      digital.constellation_soft_decoder_cf(con.base()),
      blocks.null_sink(gr.sizeof_float))

top.run()
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