ppc/pnv: check for OPAL firmware file presence

and exit before uselessly trying to load it if the file does not
exists.

Issue discovered by Coverity Scan.
Signed-off-by: NCédric Le Goater <clg@kaod.org>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/ppc/pnv.c

@@ -570,10 +570,14 @@ static void ppc_powernv_init(MachineState *machine)
     }
 
     fw_filename = qemu_find_file(QEMU_FILE_TYPE_BIOS, bios_name);
+    if (!fw_filename) {
+        error_report("Could not find OPAL firmware '%s'", bios_name);
+        exit(1);
+    }
 
     fw_size = load_image_targphys(fw_filename, FW_LOAD_ADDR, FW_MAX_SIZE);
     if (fw_size < 0) {
-        error_report("Could not load OPAL '%s'", fw_filename);
+        error_report("Could not load OPAL firmware '%s'", fw_filename);
         exit(1);
     }
     g_free(fw_filename);