hcrypto: w32crypto crypt provider handle leak

_hc_CryptProvider() returns a global handle to a Win32 Crypt Provider.
If the global handle is NULL, then a handle is allocated.  Unfortunately,
due to a coding mistake the global handle variable, g_cryptprovider, was
never set and a new handle was allocated with each call.

Refactor the function to ensure that the global handle is the value
that is returned.  Use NULL instead of 0 for pointer assignment.

Change-Id: If1ef3aa19cbd1d51860370db24c086e86922ff0d