Discovered this pre-existing problem while testing GH-10682.
Note: this problem existed *before* that PR.
* Not all paths throw a hierarchy request error
* xmlFreeNode must be used instead of xmlFree for the fragment to also
free its children.
* Free up nodes that couldn't be added when xmlAddChild fails.
I unified the error handling code that's exactly the same with a goto to
prevent at least some of such problems in the future.
Closes GH-10981.
Niels Dossche
0579beb842