Fix the following clang 3.6.0 warning in contrib/amd/hlfsd/homedir.c:

contrib/amd/hlfsd/homedir.c:497:8: error: address of array 'buf' will
always evaluate to 'true' [-Werror,-Wpointer-bool-conversion]

if (!buf || buf[0] == '\0')
    ~^~~

In the affected function, 'buf' is declared as an array of char, so it
can never be null. Remove the unecessary check.