virterror: Make SetError work if no previous error was set

virGetLastError returns NULL if no error has been set, not on
allocation error like virSetError assumed. Use virLastErrorObject
instead. This fixes virSetError when no error is currently stored.

src/util/virterror.c

@@ -304,7 +304,7 @@ int
 virSetError(virErrorPtr newerr)
 {
     virErrorPtr err;
-    err = virGetLastError();
+    err = virLastErrorObject();
     if (!err)
         return -1;