gststructure: Fix gst_structure_take ownership handling

The old code would leave a dangling pointer in oldstr_ptr if two threads attempted to take the same structure into the same location at the same time:

1. First oldstr == newstr check (before the loop) fails.
2. Compare-and-exchange fails, due to a second thread completing the same gst_structure_take.
3. Second oldstr == newstr check (in the loop) succeeds, loop breaks.
4. oldstr check succeeds, old structure gets freed.
5. oldstr_ptr now contains a dangling pointer.

This shouldn't happen in code that handles ownership sanely, so check that we don't try to do this and complain loudly.

Also simplify the function by using a do-while loop, like gst_mini_object_take.