patch 8.2.0967: unnecessary type casts for vim_strnsave()

Problem:    Unnecessary type casts for vim_strnsave().
Solution:   Remove the type casts.
diff --git a/src/fileio.c b/src/fileio.c
index 3896ba8..7b60529 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -2724,7 +2724,7 @@
     }
     else
     {
-	r = vim_strnsave(*pp, (int)(p - *pp));
+	r = vim_strnsave(*pp, p - *pp);
 	*pp = p + 1;
 	if (r != NULL)
 	{