glib/tests/enviroment.c: Fix running on Windows

Don't attempt to insert environmental variables in the hash table during
the test listenv that is an empty string, as GetEnvironmentStringsW() also
returns special enviroment variables which have empty strings as their
variable names, at least on Windows 7 and 8.

https://bugzilla.gnome.org/show_bug.cgi?id=711047
gio/tests/memory-output-stream.c: Avoid an uninitialized variable

Be a little bit more careful in regards to initializing a primitive type
variable before passing it by reference, as it could have random stuff
in the variable's address depending on the CRT, such as MSVCR110.DLL,
causing random, invalid stuff being written in that address.

This will fix this test when built with Visual Studio 2012.

https://bugzilla.gnome.org/show_bug.cgi?id=711047