mISDN: Fix memory leak in dsp_pipeline_build()

In the Linux kernel, the following vulnerability has been resolved:

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.