global-functions: $FileGet: mitigate race with file properties

RouterOS is suffering a race condition, where a file exists, but its
properties are not (yet) available. This is handled in $WaitForFile.

This passes an interval of zero to $WaitForFile, as does not wait for
the file to exist, but wants to avoid the race only.

global-functions.rsc

@@ -559,6 +559,12 @@
 :set FileGet do={
   :local FileName [ :tostr $1 ];
 
+  :global WaitForFile;
+
+  :if ([ $WaitForFile $FileName 0s ] = false) do={
+    :return false;
+  }
+
   :local FileVal false;
   :do {
     :set FileVal [ /file/get $FileName ];