Better file-existence test on Windows.

Windows, annoyingly, doesn't seem to have any bit flag in
GetFileAttributes which distinguishes a regular file (which can be
truncated destructively) from a named pipe (which can't).

Fortunately, I'm saved from needing one by the policy I came up with
in the previous commit, in which I don't bother to ask about
truncating a file if it already has zero length, because it would make
no difference anyway. Named pipes do show up as zero-length in
GetFileAttributesEx, so they get treated like zero-length regular
files by this change and it's all good.

windows/winmisc.c

@@ -590,11 +590,33 @@ FontSpec *fontspec_deserialise(void *vdata, int maxsize, int *used)
 
 int open_for_write_would_lose_data(const Filename *fn)
 {
-    FILE *fp;
+    WIN32_FILE_ATTRIBUTE_DATA attrs;
-    if ((fp = f_open(fn, "r", FALSE)) != NULL) {
+    if (!GetFileAttributesEx(fn->path, GetFileExInfoStandard, &attrs)) {
-        fclose(fp);
+        /*
-        return TRUE;
+         * Generally, if we don't identify a specific reason why we
-    } else {
+         * should return true from this function, we return false, and
+         * let the subsequent attempt to open the file for real give a
+         * more useful error message.
+         */
         return FALSE;
     }
+    if (attrs.dwFileAttributes & (FILE_ATTRIBUTE_DEVICE |
+                                  FILE_ATTRIBUTE_DIRECTORY)) {
+        /*
+         * File is something other than an ordinary disk file, so
+         * opening it for writing will not cause truncation. (It may
+         * not _succeed_ either, but that's not our problem here!)
+         */
+        return FALSE;
+    }
+    if (attrs.nFileSizeHigh == 0 && attrs.nFileSizeLow == 0) {
+        /*
+         * File is zero-length (or may be a named pipe, which
+         * dwFileAttributes can't tell apart from a regular file), so
+         * opening it for writing won't truncate any data away because
+         * there's nothing to truncate anyway.
+         */
+        return FALSE;
+    }
+    return TRUE;
 }