patch 9.0.1593: MS-Windows: assert error when compiled with debug mode

Problem:    MS-Windows: assert error when compiled with debug mode.
Solution:   Adjust arguments to setvbuf(). (Ken Takata, closes #12467)
diff --git a/src/alloc.c b/src/alloc.c
index e3e6d04..fa92c56 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -580,6 +580,7 @@
 # ifdef FEAT_EVAL
     free_resub_eval_result();
 # endif
+    free_vbuf();
 }
 #endif