malloc-fail: Fix memory leak in xmlSAX2ExternalSubset

Found with libFuzzer, see #344.
diff --git a/SAX2.c b/SAX2.c
index 3d75751..44c6b8c 100644
--- a/SAX2.c
+++ b/SAX2.c
@@ -415,6 +415,7 @@
 	                 xmlMalloc(5 * sizeof(xmlParserInputPtr));
 	if (ctxt->inputTab == NULL) {
 	    xmlSAX2ErrMemory(ctxt, "xmlSAX2ExternalSubset");
+            xmlFreeInputStream(input);
 	    ctxt->input = oldinput;
 	    ctxt->inputNr = oldinputNr;
 	    ctxt->inputMax = oldinputMax;