mISDN: Fix memory leak in dsp_pipeline_build()

[ Upstream commit c6a502c2 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>