libxl: fix potential double free in libxlDriverGetDom0MaxmemConf

Commit 4ab0c959 fixed a memory leak in libxlDriverGetDom0MaxmemConf
but introduced a potential double free of mem_tokens

*** Error in `/usr/sbin/libvirtd': double free or corruption (out):
    0x00007fffc808cfd0 ***

Avoid double free by setting mem_tokens to NULL after calling
virStringListFree.

src/libxl/libxl_conf.c

@@ -1623,6 +1623,7 @@ libxlDriverGetDom0MaxmemConf(libxlDriverConfigPtr cfg,
             }
         }
         virStringListFree(mem_tokens);
+        mem_tokens = NULL;
     }
 
  physmem: