Added negative check for get_image_size()

This patch adds check for negative return value from get_image_size(),
where it is missing. It avoids unnecessary two function calls.

Signed-off-by: Zhou Jie <zhoujie2011@cn.fujitsu.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/ppc/spapr.c

@@ -1842,6 +1842,10 @@ static void ppc_spapr_init(MachineState *machine)
         exit(1);
     }
     spapr->rtas_size = get_image_size(filename);
+    if (spapr->rtas_size < 0) {
+        error_report("Could not get size of LPAR rtas '%s'", filename);
+        exit(1);
+    }
     spapr->rtas_blob = g_malloc(spapr->rtas_size);
     if (load_image_size(filename, spapr->rtas_blob, spapr->rtas_size) < 0) {
         error_report("Could not load LPAR rtas '%s'", filename);