patch 8.1.2378: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/evalbuffer.c b/src/evalbuffer.c
index 00185b1..a475165 100644
--- a/src/evalbuffer.c
+++ b/src/evalbuffer.c
@@ -825,7 +825,7 @@
 restore_buffer(bufref_T *save_curbuf)
 {
     unblock_autocmds();
-    /* Check for valid buffer, just in case. */
+    // Check for valid buffer, just in case.
     if (bufref_valid(save_curbuf))
     {
 	--curbuf->b_nwindows;