Fix a bug where xlen larger than 0x7fff was rejected (#1334)

Backported from 81bce1a

Author: xstefank

okio/src/main/java/okio/GzipSource.java

@@ -127,7 +127,7 @@ private void consumeHeader() throws IOException {
     if (((flags >> FEXTRA) & 1) == 1) {
       source.require(2);
       if (fhcrc) updateCrc(source.buffer(), 0, 2);
-      int xlen = source.buffer().readShortLe();
+      int xlen = source.buffer().readShortLe() & 0xffff;
       source.require(xlen);
       if (fhcrc) updateCrc(source.buffer(), 0, xlen);
       source.skip(xlen);

okio/src/test/java/okio/GzipSourceTest.java

@@ -15,9 +15,11 @@
  */
 package okio;
 
+import org.junit.Test;
+
+import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.util.zip.CRC32;
-import org.junit.Test;
 
 import static okio.Util.UTF_8;
 import static org.junit.Assert.assertEquals;
@@ -182,6 +184,18 @@ private void assertGzipped(Buffer gzipped) throws IOException {
     }
   }
 
+  @Test public void extraLongXlen() throws Exception {
+    int xlen = 0xffff;
+    Buffer gzippedSource = new Buffer()
+        .write(gzipHeaderWithFlags((byte) 0x04));
+    gzippedSource.writeShort((short) xlen);
+    gzippedSource.write(new byte[xlen]);
+    gzippedSource.write(ByteString.decodeHex("f3c8540400dac59e7903000000"));
+
+    Buffer gunzipped = gunzip(gzippedSource);
+    assertEquals("Hi!", gunzipped.readUtf8());
+  }
+
   private ByteString gzipHeaderWithFlags(byte flags) {
     byte[] result = gzipHeader.toByteArray();
     result[3] = flags;