Fix bad-char handling in utf-8 streaming streams. Also add test.

src/unicode.h

Index: src/unicode.h
diff --git a/src/unicode.h b/src/unicode.h
index c8f55dab3c89a6ef4c9333fcf59fdad09dfeb6b9..1299a8ff9afeeb9aa33c01a16276fe6fd8ab7c49 100644
--- a/src/unicode.h
+++ b/src/unicode.h
@@ -161,6 +161,7 @@ class Utf8 {
   typedef uint32_t Utf8IncrementalBuffer;
   static uchar ValueOfIncremental(byte next_byte,
                                   Utf8IncrementalBuffer* buffer);
+  static uchar ValueOfIncrementalFinish(Utf8IncrementalBuffer* buffer);
 
   // Excludes non-characters from the set of valid code points.
   static inline bool IsValidCharacter(uchar c);