Fix use-after-free in libxml2 reader wrapper (AXIS2C-1222)

The ctx pointer was being freed before xmlTextReaderClose() was called.
This caused the close_input_callback to receive an already-freed pointer,
resulting in use-after-free errors detected by valgrind.

Fix by reordering: close the reader first (which invokes the callback
while ctx is still valid), then free ctx.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>