fdisk(8): check result of the getdevpath(3)

fdisk(8): check result of the getdevpath(3)

We are using the getdevpath(3) function in the fdisk.c As described
in the manual:

getdevpath() returns a pointer to a malloc()'d path on success and NULL
on failure.  If a failure occurs errno will be set appropriately.

So, we need to check the result of the getdevapth() call.