rework the patch to avoid some ABI issue with people allocating entities

* include/libxml/entities.h entities.c SAX2.c parser.c: rework
  the patch to avoid some ABI issue with people allocating
  entities structure directly
Daniel

svn path=/trunk/; revision=3773
diff --git a/SAX2.c b/SAX2.c
index a813960..80cd1bd 100644
--- a/SAX2.c
+++ b/SAX2.c
@@ -580,7 +580,8 @@
 	    return(NULL);
 	}
 	ret->owner = 1;
-	ret->checked = 1;
+	if (ret->checked == 0)
+	    ret->checked = 1;
     }
     return(ret);
 }