segfault on ENVIRON access (5.1.1 but NOT in latest git)

[Mathieu Bivert]

Hello,

  I've hit a systematically reproducible segfault on ENVIRON access
on gawk-5.1.1, but not anymore with the latest git commit (381d737c).

I haven't looked too deep into the changelog/mailing list to see if it's
actually relevant; I prefer to notify you, just in case there's something
else lurking around.

Segfault occurs, among others, when **environ starts with a variable
named 0 (zero):

    % uname -a
    Linux earth 5.17.9-arch1-1 #1 SMP PREEMPT Wed, 18 May 2022 17:30:11
+0000 x86_64 GNU/Linux
    % cat test.c && cc test.c && echo | ./a.out
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <string.h>

    extern char **environ;

    char *gawk = "/home/mb/src/gawk-5.1.1/gawk";

    int main(void)
    {
        char *argv[] = {
            "gawk",
            "{\n\
                for (k in ENVIRON)\n\
                    printf(\"ENV[%s] = %s\\n\", k, ENVIRON[k])\n\
            }",
            NULL
        };

        /* assuming environ contains at least one variable */
        environ[0] = strdup("0=whatever");
        if (environ[0] == NULL) {
            perror("strdup()");
            return -1;
        }

        execv(gawk, argv);
        return 0;
    }
    gawk: cmd. line:2: (FILENAME=- FNR=1) fatal error: internal error:
segfault
    Aborted (core dumped)

Cheers,