[builds/unix, builds/vms] Standardize `mmap` failure.

* builds/unix/ftsystem.c (FT_Stream_Open): Check for MAP_FAILED.
* builds/vms/ftsystem.c (FT_Stream_Open): Ditto.

This should cover https://savannah.nongnu.org/patch/?5909 as well.

builds/unix/ftsystem.c

@@ -313,8 +313,7 @@
                                           file,
                                           0 );
 
-    /* on some RTOS, mmap might return 0 */
-    if ( (long)stream->base != -1 && stream->base != NULL )
+    if ( stream->base != MAP_FAILED )
       stream->close = ft_close_stream_by_munmap;
     else
     {

builds/vms/ftsystem.c

@@ -246,7 +246,7 @@
                                           file,
                                           0 );
 
-    if ( (long)stream->base == -1 )
+    if ( stream->base == MAP_FAILED )
     {
       FT_ERROR(( "FT_Stream_Open:" ));
       FT_ERROR(( " could not `mmap' file `%s'\n", filepathname ));