Correctly reset offset when fail lazy because of too long frame. (#8257)

Motivation:

We need to reset the offset to 0 when we fail lazy because of a too long frame.

Modifications:

- Reset offset
- Add testcase

Result:

Fixes https://github.com/netty/netty/issues/8256.

codec/src/main/java/io/netty/handler/codec/LineBasedFrameDecoder.java

@@ -137,6 +137,8 @@ public class LineBasedFrameDecoder extends ByteToMessageDecoder {
             } else {
                 discardedBytes += buffer.readableBytes();
                 buffer.readerIndex(buffer.writerIndex());
+                // We skip everything in the buffer, we need to set the offset to 0 again.
+                offset = 0;
             }
             return null;
         }

codec/src/test/java/io/netty/handler/codec/LineBasedFrameDecoderTest.java

@@ -185,4 +185,27 @@ public class LineBasedFrameDecoderTest {
         buf.release();
         buf2.release();
     }
+
+    @Test
+    public void testNotFailFast() throws Exception {
+        EmbeddedChannel ch = new EmbeddedChannel(new LineBasedFrameDecoder(2, false, false));
+        assertFalse(ch.writeInbound(wrappedBuffer(new byte[] { 0, 1, 2 })));
+        assertFalse(ch.writeInbound(wrappedBuffer(new byte[]{ 3, 4 })));
+        try {
+            ch.writeInbound(wrappedBuffer(new byte[] { '\n' }));
+            fail();
+        } catch (TooLongFrameException expected) {
+            // Expected once we received a full frame.
+        }
+        assertFalse(ch.writeInbound(wrappedBuffer(new byte[] { '5' })));
+        assertTrue(ch.writeInbound(wrappedBuffer(new byte[] { '\n' })));
+
+        ByteBuf expected = wrappedBuffer(new byte[] { '5', '\n' });
+        ByteBuf buffer = ch.readInbound();
+        assertEquals(expected, buffer);
+        expected.release();
+        buffer.release();
+
+        assertFalse(ch.finish());
+    }
 }