patch 8.2.4535: filename modifer ":8" removes the filename

Problem:    Filename modifer ":8" removes the filename.
Solution:   Use strncpy() instead of vim_strncpy(). (Christian Brabandt,
            closes #9918, closes #8600)
diff --git a/src/filepath.c b/src/filepath.c
index 7a5dae4..f0da60f 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -186,7 +186,7 @@
 	    // unless get_short_pathname() did its work in-place.
 	    *fname = *bufp = save_fname;
 	    if (short_fname != save_fname)
-		vim_strncpy(save_fname, short_fname, len);
+		STRNCPY(save_fname, short_fname, len);
 	    save_fname = NULL;
 	}