[PATCH][4.15] gnttab: work around "may be used uninitialized" warning

[Jan Beulich]

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: 52531c734ea1 ("xen/gnttab: Rework resource acquisition")
Signed-off-by: Jan Beulich <jbeul...@suse.com>

--- a/xen/common/grant_table.c
+++ b/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 )