Avoid Coverity NUL termination warning about strncpy() by using
memcpy() instead. It's probably a bit more optimal in this case
anyway. [1]

The program logic leading up to the creation of the strncpy/memcpy
destination buffer is a bit hairy. Add a call to assert() to make
it clear what is happening here and detect any potential buffer
overruns in the future.

Check a couple syscall error returns. Ignore the EEXIST error from
link() to preserve existing behavior. [2] [3]

Reported by: Coverity
CID: 1009659 [1], 1009349 [2], 1009350 [3]