[CVE-2019-20388] Fix memory leak in xmlSchemaValidateStream

[CVE-2019-20388] Fix memory leak in xmlSchemaValidateStream

When ctxt->schema is NULL, xmlSchemaSAXPlug->xmlSchemaPreRun
alloc a new schema for ctxt->schema and set vctxt->xsiAssemble
to 1. Then xmlSchemaVStart->xmlSchemaPreRun initialize
vctxt->xsiAssemble to 0 again which cause the alloced schema
can not be freed anymore.

Found with libFuzzer.

Change-Id: I19755ffa6ff031a6d5ba2b7daa82ad1b8a3b9362
Signed-off-by: Zhipeng Xie <xiezhipeng1@huawei.com>
Signed-off-by: DongHun Kwak <dh0128.kwak@samsung.com>