Roll libxml to 2.9.3

third_party/libxml/src/dict.c

Index: third_party/libxml/src/dict.c
diff --git a/third_party/libxml/src/dict.c b/third_party/libxml/src/dict.c
index 5f71d55d736f7d752c71ae720e5d3d6e487b493b..8c8f93148b6de6980b8fe62f2b89865e9ab9ee1c 100644
--- a/third_party/libxml/src/dict.c
+++ b/third_party/libxml/src/dict.c
@@ -486,7 +486,10 @@ xmlDictComputeFastQKey(const xmlChar *prefix, int plen,
 	value += 30 * (*prefix);
 
     if (len > 10) {
-        value += name[len - (plen + 1 + 1)];
+        int offset = len - (plen + 1 + 1);
+	if (offset < 0)
+	    offset = len - (10 + 1);
+	value += name[offset];
         len = 10;
 	if (plen > 10)
 	    plen = 10;