mapfile doesn't accept input from a pipe

[Keith Thompson]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2 -Wno-parentheses -Wno-format-security
uname output: Linux bomb20 4.8.0-46-generic #49-Ubuntu SMP Fri Mar 31
13:57:14 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-unknown-linux-gnu

Bash Version: 4.4
Patch Level: 12
Release Status: maint

Description:
        The "mapfile" command works correctly if stdin is redirected
from a file, but not if it's from a pipe.
        Demonstrated with several versions including the latest
bash-snap-20170626

Repeat-By:
        This script demonstrates the problem:
        __CUT_HERE__
        #!/bin/bash

        printf 'one\ntwo\nthree\n' > /tmp/input.txt

        mapfile REDIRECT < /tmp/input.txt
        cat /tmp/input.txt | mapfile PIPE

        echo "\$REDIRECT has ${#REDIRECT[@]} elements"
        echo "\$PIPE     has ${#PIPE[@]} elements"

        if [ ${#REDIRECT[@]} -eq 3 ] && [ ${#PIPE[@]} -eq 3 ] ; then
            echo PASS
            exit 0
        else
            echo FAIL
            exit 1
        fi
        __AND_HERE__