gio/tests/memory-output-stream.c: Avoid an uninitialized variable

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