Fix copy paste error

Author: makuto

runtime/FileUtilities.cake

@@ -124,7 +124,7 @@
   (rewind in-file)
   (var-cast-to out-buffer (* char) (malloc (+ 1 file-size)))
   (fread out-buffer file-size 1 in-file)
-  (set (at file-size contents-buffer) 0)
+  (set (at file-size out-buffer) 0)
   (return out-buffer))
 
 (defun write-string (out-file (* FILE) out-string (* (const char)))