media: vidtv: mux: Add check and kfree for kstrdup

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

media: vidtv: mux: Add check and kfree for kstrdup

Add check for the return value of kstrdup() and return the error

if it fails in order to avoid NULL pointer dereference.

Moreover, use kfree() in the later error handling in order to avoid

memory leak.