tools/libxl: Fix uninitialised variable in libxl__write_stub_dmargs()

Various version of gcc, when compiling with -Og, complain:

  libxl_dm.c: In function ‘libxl__write_stub_dmargs’:
  libxl_dm.c:2166:16: error: ‘dmargs’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
               rc = libxl__xs_write_checked(gc, t, path, dmargs);
               ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It isn't actually used while uninitialised, but only because of how the
is_linux_stubdom checks line up.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>

tools/libs/light/libxl_dm.c

@@ -2101,7 +2101,7 @@ static int libxl__write_stub_dmargs(libxl__gc *gc,
 {
     struct xs_permissions roperm[2];
     xs_transaction_t t = XBT_NULL;
-    char *dmargs;
+    char *dmargs = NULL;
     int rc;
 
     roperm[0].id = 0;