accessing ENVIRON causes SIGFPE when first var is numeric

[David Arroyo]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2 -DNDEBUG
uname output: Linux dorcas 5.15.43 #1 SMP 1 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Gawk Version: 5.1.60 (HEAD)

Attestation:
        I have read 
https://www.gnu.org/software/gawk/manual/html_node/Bugs.html.
        Yes

Description:

Since commit 7d19cbd5, if the first environment variable is a
number, gawk will treat ENVIRON as a cint_array instead of a
str_array. You can reproduce this with the following command
line:

        env -i \
          $(which env) 0=foo x=bar \
          $(which gawk) 'BEGIN { ENVIRON["x"] }' /dev/null

Will yield the error:

        gawk: cmd. line:1: fatal: floating point exception

 at str_array.c:607 

And I can see from a debugger that after the first iteration
of load_environ, ENVIRON_node->array_funcs becomes
cint_array_func. This issue does not occure when using
the --posix flag.

I ran into this error on my personal computer running GNU
Guix, where the first variable in my environment is

        0=/home/me/bin/startw

I am not sure if this is common, as I do not see such an
env var on a debian 10 system I had handy, and I have not
tracked down what exactly sets that variable. But it seems
undesirable for variations in user environment variables
to trigger a crash.

Fix:

The patch below hardcodes ENVIRON_node to use the
str_array_func family of functions. I don't know if that is
the best way to fix the issue, but it helps to illustrate what's
happening. I could not reproduce this issue using an 
arbitrary array.

---
 main.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/main.c b/main.c
index 2cdd1e2d..3f48ddfb 100644
--- a/main.c
+++ b/main.c
@@ -922,6 +922,7 @@ load_environ()
        been_here = true;

        ENVIRON_node = install_symbol(estrdup("ENVIRON", 7), Node_var_array);
+       ENVIRON_node->array_funcs = &str_array_func;
        for (i = 0; environ[i] != NULL; i++) {
                static char nullstr[] = "";

--