Check charset of fragmented subject

app/src/main/java/eu/faircode/email/CharsetHelper.java

@@ -30,6 +30,7 @@ import java.nio.charset.Charset;
 import java.nio.charset.CharsetDecoder;
 import java.nio.charset.CodingErrorAction;
 import java.nio.charset.StandardCharsets;
+import java.nio.charset.UnsupportedCharsetException;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
@@ -76,24 +77,23 @@ public class CharsetHelper {
     }
 
     static boolean isUTF8(byte[] octets) {
-        CharsetDecoder utf8Decoder = StandardCharsets.UTF_8.newDecoder()
-                .onMalformedInput(CodingErrorAction.REPORT)
-                .onUnmappableCharacter(CodingErrorAction.REPORT);
-        try {
-            utf8Decoder.decode(ByteBuffer.wrap(octets));
-            return true;
-        } catch (CharacterCodingException ex) {
-            Log.w(ex);
-            return false;
-        }
+        return isValid(octets, StandardCharsets.UTF_8);
     }
 
     static boolean isUTF16(byte[] octets) {
-        CharsetDecoder utf8Decoder = StandardCharsets.UTF_16.newDecoder()
+        return isValid(octets, StandardCharsets.UTF_16);
+    }
+
+    static Boolean isValid(byte[] octets, String charset) {
+        return isValid(octets, Charset.forName(charset));
+    }
+
+    static boolean isValid(byte[] octets, Charset charset) {
+        CharsetDecoder decoder = charset.newDecoder()
                 .onMalformedInput(CodingErrorAction.REPORT)
                 .onUnmappableCharacter(CodingErrorAction.REPORT);
         try {
-            utf8Decoder.decode(ByteBuffer.wrap(octets));
+            decoder.decode(ByteBuffer.wrap(octets));
             return true;
         } catch (CharacterCodingException ex) {
             Log.w(ex);

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -2880,6 +2880,11 @@ public class MessageHelper {
                 try {
                     byte[] b1 = decodeWord(p1.text, p1.encoding, p1.charset);
                     byte[] b2 = decodeWord(p2.text, p2.encoding, p2.charset);
+                    if (CharsetHelper.isValid(b1, p1.charset) && CharsetHelper.isValid(b2, p2.charset)) {
+                        p++;
+                        continue;
+                    }
+
                     byte[] b = new byte[b1.length + b2.length];
                     System.arraycopy(b1, 0, b, 0, b1.length);
                     System.arraycopy(b2, 0, b, b1.length, b2.length);