virhook: Resolve Coverity NULL_RETURNS

Coverity complains that many other callers to return err from
virGetLastError() will check if err is not NULL before dereferencing
it.  Just do the same here for safety.

src/util/virhook.c

@@ -300,7 +300,8 @@ virHookCall(int driver,
     if (ret < 0) {
         /* Convert INTERNAL_ERROR into known error.  */
         virErrorPtr err = virGetLastError();
-        virReportError(VIR_ERR_HOOK_SCRIPT_FAILED, "%s", err->message);
+        virReportError(VIR_ERR_HOOK_SCRIPT_FAILED, "%s",
+                       err ? err->message : _("unknown error"));
     }
 
     virCommandFree(cmd);