Avoid compilation warnings in w32.c

* src/w32.c (unsetenv, readlink): Use memcpy instead of
strncpy, to avoid a compiler warning about calculating the
bound of the copy.
1 job for master in 73 minutes and 25 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #1578
test-all

01:13:25