gnttab: work around "may be used uninitialized" warning

Sadly I was wrong to suggest dropping vaddrs' initializer during review
of v2 of the patch introducing this code. gcc 4.3 can't cope.

Fixes: 52531c7 ("xen/gnttab: Rework resource acquisition")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>

xen/common/grant_table.c

@@ -4026,7 +4026,7 @@ int gnttab_acquire_resource(
     struct grant_table *gt = d->grant_table;
     unsigned int i, final_frame;
     mfn_t tmp;
-    void **vaddrs;
+    void **vaddrs = NULL;
     int rc = -EINVAL;
 
     if ( !nr_frames )
@@ -4055,6 +4055,17 @@ int gnttab_acquire_resource(
         break;
     }
 
+    /*
+     * Some older toolchains can't spot that vaddrs won't remain uninitialized
+     * on non-error paths, and hence it needs setting to NULL at the top of the
+     * function.  Leave some runtime safety.
+     */
+    if ( !vaddrs )
+    {
+        ASSERT_UNREACHABLE();
+        rc = -ENODATA;
+    }
+
     /* Any errors?  Bad id, or from growing the table? */
     if ( rc )
         goto out;