patch 8.0.0518: bad fold text when a multi-byte char has a zero byte

Problem:    Storing a zero byte from a multi-byte character causes fold text
            to show up wrong.
Solution:   Avoid putting zero in ScreenLines. (Christian Brabandt,
            closes #1567)
diff --git a/src/screen.c b/src/screen.c
index 82c5ba5..6a7284b 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -2697,12 +2697,15 @@
 	    {
 		ScreenLinesUC[off + col] = fill_fold;
 		ScreenLinesC[0][off + col] = 0;
+                ScreenLines[off + col] = 0x80; /* avoid storing zero */
 	    }
 	    else
 		ScreenLinesUC[off + col] = 0;
+	    col++;
 	}
+	else
 #endif
-	ScreenLines[off + col++] = fill_fold;
+	    ScreenLines[off + col++] = fill_fold;
     }
 
     if (text != buf)