GVariantTypeInfo: fix thread safety bug

GVariantTypeInfo: fix thread safety bug

Issue caught by Michael Meeks.  This patch based on his.

GVariantTypeInfo was dropping the reference count on the TypeInfo
structure before removing it from the hash table.  This means that
another thread could come along and grab the value from the hash table
in the meantime.

Solve this by holding the lock on the table before dropping the
reference.

Also: move the hash table initialisation inside of the lock to remove
the standard double-initialisation race plus a more insidious issue
caused by the fact that we free the hash table once it becomes empty.