global: reset global state on shutdown without threading

global: reset global state on shutdown without threading

When threading is not enabled for libgit2, we keep global state
in a simple static variable. When libgit2 is shut down, we clean
up the global state by freeing the global state's dynamically
allocated memory. When libgit2 is built with threading, we
additionally free the thread-local storage and thus completely
remove the global state. In a non-threaded build, though, we
simply leave the global state as-is, which may result in an error
upon reinitializing libgit2.

Fix the issue by zeroing out the variable on a shutdown, thus
returning it to its initial state.