mISDN: Fix memory leak in dsp_pipeline_build()

dsp_pipeline_build() allocates dup pointer by kstrdup(cfg),
but then it updates dup variable by strsep(&dup, "|").
As a result when it calls kfree(dup), the dup variable contains NULL.

Found by Linux Driver Verification project (linuxtesting.org) with SVACE.

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Fixes: 960366cf

 ("Add mISDN DSP")
Signed-off-by: David S. Miller <davem@davemloft.net>