Fix a double free in v2i_NAME_CONSTRAINTS()

Fix a double free in v2i_NAME_CONSTRAINTS()

a2i_GENERAL_NAME() modifies and returns the out argument that was
passed in unless out == NULL, in which case it returns something
freshly allocated. Thus, in v2i_GENERAL_NAME_ex() we must only free
ret if out == NULL so v2i_NAME_CONSTRAINTS() can free correctly.

Issue reported by Volker Schlecht

ok jsing