Memory leak in 5.1 when redeclaring arrays

[Alexander Mescheryakov]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -march=x86-64 -mtune=generic -O2 -pipe -fno-plt 
-DDEFAULT_PATH_VALUE='/usr/local/sbin:/usr/local/bin:/usr/bin' 
-DSTANDARD_UTILS_PATH='/usr/bin' -DSYS_BASHRC='/etc/bash.bashrc' 
-DSYS_BASH_LOGOUT='/etc/bash.bash_logout' -DNON_INTERACTIVE_LOGIN_SHELLS 
uname output: Linux Valhalla 5.9.14-arch1-1 #1 SMP PREEMPT Sat, 12 Dec 2020 
14:37:12 +0000 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.1
Patch Level: 4
Release Status: release

Description:
        Redeclaring array variable leads to leaked memory in bash 5.1

    It is required to define var as `declare -a FOO=(a b c)`
    defining it simply as `FOO=(a b c)` does not introduce memory leak.

    The same actions in bash 5.0 do not lead to memory leak.

Repeat-By:
        Here is the script to reproduce the issue:

    ```
    #!/bin/bash
    
    SOURCE_FILE=/tmp/strings
    
    echo 'declare -a STRINGS=(' > "$SOURCE_FILE"
    for i in {1..100}; do
        echo 'qwertyuiopasdfghjklzxcvbnm' >> "$SOURCE_FILE"
    done
    echo ')' >> "$SOURCE_FILE"
    
    for i in {1..10000}; do
        if [ $((i%1000)) -eq 0 ]; then
            printf '%d\t%s\n' "$((i/1000))" "$(grep VmRSS /proc/$$/status)"
        fi
        source "$SOURCE_FILE"
    done
    ```

    Sample output with bash 5.1:

    1       VmRSS:     23276 kB
    2       VmRSS:     41916 kB
    3       VmRSS:     60688 kB
    4       VmRSS:     79452 kB
    5       VmRSS:     98088 kB
    6       VmRSS:    116988 kB
    7       VmRSS:    135628 kB
    8       VmRSS:    154528 kB
    9       VmRSS:    173296 kB
    10      VmRSS:    191932 kB

    For comparison the same script under bash 5.0 does not leak:

    $ bash-5.0/bash ~/test_bash_memleak.sh 
    1       VmRSS:      6124 kB
    2       VmRSS:      6204 kB
    3       VmRSS:      6204 kB
    4       VmRSS:      6204 kB
    5       VmRSS:      6204 kB
    6       VmRSS:      6204 kB
    7       VmRSS:      6204 kB
    8       VmRSS:      6204 kB
    9       VmRSS:      6204 kB
    10      VmRSS:      6204 kB