Bug 24549 – std.process.environment.get(null) segfaults

It's of course an invalid input, but a segfault isn't really expected. Tested with Phobos v2.108.1 on Linux.

The underlying issue is that `std.internal.cstring.tempCString` returns a `null` pointer if the input string is an empty string with a null pointer. This is surprising, and seems to have been done as a hack to work around issue 14980 I would not expect to get a null pointer when passing in an empty string, I'd expect an empty string. This has a seemingly "unforseen" effect on code that is accepting of a string, and blindly passes to `tempCString` for calling a C lib -- if the library function doesn't handle null pointers, or handles them differently than non-null empty strings, then it may crash or do something unexpected. The Linux `getenv` function is crashing on the null pointer (BTW, this does not happen on MacOS).

@schveiguy updated dlang/phobos pull request #9002 "Fix issue 24549" fixing this issue: - Fix bugzilla issue 24549 -- environment.get(null) segfaults on Linux https://github.com/dlang/phobos/pull/9002

dlang/phobos pull request #9002 "Fix issue 24549 - std.process.environment.get(null) segfaults" was merged into stable: - 6ea12cf3a4b2fb6e3e962a87e1954b949f349a79 by Steven Schveighoffer: Fix bugzilla issue 24549 -- environment.get(null) segfaults on Linux https://github.com/dlang/phobos/pull/9002

dlang/phobos pull request #9006 "merge stable" was merged into master: - 82e8e32b2ff546a86eb3683b1af4ecc5939f633e by Steven Schveighoffer: Fix bugzilla issue 24549 -- environment.get(null) segfaults on Linux https://github.com/dlang/phobos/pull/9006