nbd-server: handle port number string conversion errors

g_strdup_printf() tries to allocate a string large enough to hold the
string representation of the passed value and coverts the value to
string. It returns NULL if memory allocation fails, or some other error
occurs. Previously, NULL was interpreted as "modern style socket not
needed for this export" by returning zero to the caller, which was
utterly wrong.

Signed-off-by: Tuomas Räsänen <tuomasjjrasanen@tjjr.fi>