[directwrite] Use correct UTF-16 string length

Fixes https://github.com/harfbuzz/harfbuzz/issues/2474

src/hb-directwrite.cc

diff --git a/src/hb-directwrite.cc b/src/hb-directwrite.cc
index a073021..aebe9e0 100644
--- a/src/hb-directwrite.cc
+++ b/src/hb-directwrite.cc
@@ -623,7 +623,7 @@
   * but we never attempt to shape a word longer than 64K characters
   * in a single gfxShapedWord, so we cannot exceed that limit.
   */
-  uint32_t textLength = buffer->len;
+  uint32_t textLength = chars_len;
 
   TextAnalysis analysis (textString, textLength, nullptr, readingDirection);
   TextAnalysis::Run *runHead;