parser: Fix potential memory leak in xmlParseAttValueInternal

Fix memory leak in case xmlParseAttValueInternal is called with a NULL
`len` a non-NULL `alloc` argument. This static function is never called
with such arguments internally, but the misleading code should be fixed
nevertheless.

Fixes #422.

parser.c

@@ -9155,6 +9155,7 @@ xmlParseAttValueInternal(xmlParserCtxtPtr ctxt, int *len, int *alloc,
     in++;
     col++;
     if (len != NULL) {
+        if (alloc) *alloc = 0;
         *len = last - start;
         ret = (xmlChar *) start;
     } else {
@@ -9164,7 +9165,6 @@ xmlParseAttValueInternal(xmlParserCtxtPtr ctxt, int *len, int *alloc,
     CUR_PTR = in;
     ctxt->input->line = line;
     ctxt->input->col = col;
-    if (alloc) *alloc = 0;
     return ret;
 need_complex:
     if (alloc) *alloc = 1;