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.