tests/resource: Initialise gnttab before xenforeignmemory_map_resource()

It's the 'addr' input to mmap(), and currently consuming stack rubble.

Coverity-ID: 1500115
Fixes: c7a7f14b92 ("tests/resource: Extend to check that the grant frames are mapped correctly")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

tools/tests/resource/test-resource.c

@@ -24,7 +24,7 @@ static void test_gnttab(uint32_t domid, unsigned int nr_frames,
                         unsigned long gfn)
 {
     xenforeignmemory_resource_handle *res;
-    grant_entry_v1_t *gnttab;
+    grant_entry_v1_t *gnttab = NULL;
     size_t size;
     int rc;
     uint32_t refs[nr_frames], domids[nr_frames];