posix handling of mapfile: SEGFAULT

[Linda Walsh]

This was in the 2nd half of the note in the read&env+POSIX=>SEGFAULT,
but think it got missed by focus on the 1st part.

# I was doing some syntax testing and decided to try posix mode
# (as it disallows various  vague or unclear constructs)
# in the working cases yielded the same results, but
# in 2 cases it returned a SEGFAULT.

>  b= mapfile b <<< x; echo $?
0
>  declare -p b
Segmentation fault (core dumped)  

# another example of bash not returning consistent errno results
# ( |;>|  ducking while smirking)  ok, I vote for disallowing this
# type of result return.

The 2nd flavor:

>  b= mapfile b a <<< x
>  declare -p a b
bash: declare: a: not found
Segmentation fault (core dumped) 

# odd, this time it dumped on printing 'b'; still not so great
# status returning there...

# hmmm... I'm guessing that fixing these to not return
# segfaults won't be likely to get me better status
# returns for my type -P problems....*sigh*


# vitals:
> $0 --version
GNU bash, version 4.3.39(1)-release (x86_64-unknown-linux-gnu)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later 
<http://gnu.org/licenses/gpl.html>
> whence bash
bash is /bin/bash
Ishtar:/Torrents/Library/2014Q1> ldd /bin/bash
linux-vdso.so.1 (0x00007fff396eb000)
libdl.so.2 => /lib64/libdl.so.2 (0x0000003001400000)
libc.so.6 => /lib64/libc.so.6 (0x0000003000c00000)
/lib64/ld-linux-x86-64.so.2 (0x000055da52cbc000)
> uname -a
Linux Ishtar 4.1.0-Isht-Van #2 SMP PREEMPT Tue Jun 23 07:52:09 PDT 2015 
x86_64 x86_64 x86_64 GNU/Linux