malloc-fail: Handle malloc failures in xmlSchemaInitTypes

Note that this changes the return value of public function
xmlSchemaInitTypes from void to int. This shouldn't break the ABI on
most platforms.

Found when investigating #500.

include/libxml/xmlschemastypes.h

@@ -30,7 +30,7 @@ typedef enum {
     XML_SCHEMA_WHITESPACE_COLLAPSE = 3
 } xmlSchemaWhitespaceValueType;
 
-XMLPUBFUN void
+XMLPUBFUN int
 		xmlSchemaInitTypes		(void);
 XML_DEPRECATED
 XMLPUBFUN void