secret: Fix passing pointers to libvirt APIs

Inspired by one of previous commits, we can not assume that an
array passed to an API has any items, thus getting its pointer to
pass to C API like this: "&array[0]" may lead to a runtime error.
If that's the case, pass nil instead.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

secret.go

@@ -176,14 +176,19 @@ func (s *Secret) GetValue(flags uint32) ([]byte, error) {
 
 // See also https://libvirt.org/html/libvirt-libvirt-secret.html#virSecretSetValue
 func (s *Secret) SetValue(value []byte, flags uint32) error {
-	cvalue := make([]C.uchar, len(value))
+	nvalue := len(value)
+	cvalue := make([]C.uchar, nvalue)
 
-	for i := 0; i < len(value); i++ {
+	for i := 0; i < nvalue; i++ {
 		cvalue[i] = C.uchar(value[i])
 	}
 
 	var err C.virError
-	result := C.virSecretSetValueWrapper(s.ptr, &cvalue[0], C.size_t(len(value)), C.uint(flags), &err)
+	var cvaluePtr *C.uchar = nil
+	if nvalue > 0 {
+		cvaluePtr = &cvalue[0]
+	}
+	result := C.virSecretSetValueWrapper(s.ptr, cvaluePtr, C.size_t(nvalue), C.uint(flags), &err)
 
 	if result == -1 {
 		return makeError(&err)