Avoid use of uninitialized in ASN1_STRING_to_UTF8()

Avoid use of uninitialized in ASN1_STRING_to_UTF8()

A long standing failure to initialize a struct on the stack fully was
exposed by a recent refactoring. Fortunately, the uninitialized 'flag'
member is only used to decide whether or not to call freezero(NULL, 0),
so it is completely harmless. This is a first trivial fix, a better
version will be landed separately with regress.

Reported by Steffen Jaeckel, GH #760

ok beck