process.c: handle errno correctly in non-SIGCHLD path

It's a bit redundant, but we optimize for platforms with
SIGCHLD, not without.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63790 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

process.c

@@ -1114,6 +1114,8 @@ waitpid_no_SIGCHLD(struct waitpid_state *w)
                                        RUBY_UBF_PROCESS, 0);
         } while (w->ret < 0 && errno == EINTR && (RUBY_VM_CHECK_INTS(w->ec),1));
     }
+    if (w->ret == -1)
+        w->errnum = errno;
 }
 
 rb_pid_t