thread.c (consume_communication_pipe): do not retry after short read

Saves a syscall and slightly improves vm_thread_condvar1
benchmark slightly (more improvements on the way):

                         r64170        this patch
vm_thread_condvar1        0.917        1.065

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64185 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread.c

@@ -4273,6 +4273,9 @@ consume_communication_pipe(int fd)
 	result = read(fd, buff, sizeof(buff));
 	if (result > 0) {
 	    ret = TRUE;
+	    if (result < (ssize_t)sizeof(buff)) {
+		return ret;
+	    }
 	}
 	else if (result == 0) {
 	    return ret;