core: Set ctx in global_xlator to prevent NULL ptr reference

This results in a crash in STACK_UNWIND after a call bail out.
Since the call bail does not occur in the context of an xlator,
the old_THIS in STACK_UNWIND is handed a global_xlator which did
not have its ctx set, resulting in a NULL de-reference.

Signed-off-by: Shehjar Tikoo <shehjart@gluster.com>
Signed-off-by: Anand V. Avati <avati@dev.gluster.com>

BUG: 844 (mixed testing with 35 clients)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=844

libglusterfs/src/globals.c

@@ -87,6 +87,7 @@ glusterfs_this_init ()
 
         global_xlator.name = "glusterfs";
         global_xlator.type = "global";
+        global_xlator.ctx  = glusterfs_ctx;
 
         return ret;
 }