TerminalEmulator: conform to standard on handling of invalid UTF-8 se…

…quences

The Unicode standard requires that, when dealing with ill-formed UTF-8
(version 6.2, page 96):

    If the converter encounters an ill-formed UTF-8 code unit sequence
    which starts with a valid first byte, but which does not continue
    with valid successor bytes [...], it must not consume the successor
    bytes as part of the ill-formed subsequence whenever those successor
    bytes themselves constitute part of a well-formed UTF-8 code unit
    subsequence.

This implies that when we hit a byte in the input stream which cannot
fit into the sequence currently being decoded, we must attempt to decode
that byte again after resetting our decoder state.

libraries/emulatorview/src/jackpal/androidterm/emulatorview/TerminalEmulator.java

@@ -823,7 +823,11 @@ private boolean handleUTF8Sequence(byte b) {
                 mUTF8ToFollow = 0;
                 mUTF8ByteBuffer.clear();
                 emit(UNICODE_REPLACEMENT_CHAR);
-                return true;
+
+                /* The Unicode standard (section 3.9, definition D93) requires
+                 * that we now attempt to process this byte as though it were
+                 * the beginning of another possibly-valid sequence */
+                return handleUTF8Sequence(b);
             }
 
             mUTF8ByteBuffer.put(b);