[php-src] Issue #21797: phar: NULL dereference in Phar::webPhar() when SCRIPT_NAME is absent from SAPI environment

Issue: https://github.com/php/php-src/issues/21797
Author: iliaal

## Description

`Phar::webPhar()` calls `sapi_getenv("SCRIPT_NAME", ...)` and passes the result directly to `strstr()` without checking for NULL. When the SAPI environment does not provide `SCRIPT_NAME` (e.g. a misconfigured FastCGI upstream), `sapi_getenv` returns NULL and the `strstr` call segfaults.

## Affected code

`ext/phar/phar_object.c`, `PHP_METHOD(Phar, webPhar)`:

```c
testit = sapi_getenv("SCRIPT_NAME", sizeof("SCRIPT_NAME")-1);
if (!(pt = strstr(testit, basename))) {   // NULL dereference if testit == NULL
    efree(testit);
    goto finish;
}
```

## Trigger conditions

FastCGI deployment (nginx, Caddy, LiteSpeed) where `SCRIPT_NAME` is not forwarded in the FastCGI params block. This is an atypical but possible misconfiguration. Not reachable via `php-cgi` invoked directly, since CGI SAPI derives `request_uri` from `SCRIPT_NAME` and returns early before this code is reached.

## Expected behavior

`webPhar()` should handle a missing `SCRIPT_NAME` gracefully (treat it as non-matching and fall through to the `finish` label).

## Fix

Add a NULL guard immediately after the `sapi_getenv` call:

```c
testit = sapi_getenv("SCRIPT_NAME", sizeof("SCRIPT_NAME")-1);
if (!testit) {
    goto finish;
}
if (!(pt = strstr(testit, basename))) {
    efree(testit);
    goto finish;
}
```