windows: always initialize uv_process_t

The unix implementation of uv_spawn always starts out with a
uv__handle_init, but the windows implementation sometimes bails out
early before calling uv__handle_init. This means that uv_close on a
failed uv_spawn will always succeed on unix but sometimes fail on
windows.

This commit lifts the initialization of the uv_process_t above all of
the error checking to ensure that uv_close will always work when
uv_spawn returns an error.