bash -- out-of-bounds memory access in parser

RedHat security team reports:

It was discovered that the fixed-sized redir_stack could be forced
to overflow in the Bash parser, resulting in memory corruption, and
possibly leading to arbitrary code execution when evaluating
untrusted input that would not otherwise be run as code.

An off-by-one error was discovered in the way Bash was handling
deeply nested flow control constructs. Depending on the layout of
the .bss segment, this could allow arbitrary execution of code that
would not otherwise be executed by Bash.