bash: Fix file-substitution error-handling bug

This is part of a patch that's been upstream for a while but hasn't yet
been released.  The bug is causing some downstream difficulties, so a
local patch to tide us over until the next release makes things a bit
easier.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit bf384d66)
Signed-off-by: Steve Sakoman <steve@sakoman.com>