When xvi runs out of memory, you can't save the buffer

When xvi runs out of RAM (like in issue #130) you can't write the file.
To ":w foo" it says "Cannot open preserve file /tmp/#foo.tmp", also if you force the write ``:w! foo"

If you bypass the preserve file when forcing:

--- a/src/fileio.c
+++ b/src/fileio.c
@@ -612,7 +612,9 @@ bool_t     force;
     /*
      * Preserve the buffer here so if the write fails it will at
-     * least have been saved.
+     * least have been saved.  The exception when forcing is to
+     * improve the chances of being able to save the buffer
+     * when xvi has run out of memory.
      */
-    if (need_preserve && !preservebuf(window)) {
+    if (!force && need_preserve && !preservebuf(window)) {
        return(FALSE);
     }

It says "Can't write foo" instead because the fopen() fails for lack of RAM.
