crypt: Streamline get_asymmetric_key().

crypt: Streamline get_asymmetric_key().

The clang analyzer says

crypt.c:202:9: warning: Function call argument is an uninitialized value
        ret2 = para_munmap(map, map_size);
               ^                ~~~~~~~~
And right it is. This is not a bug though as map_size is only undefined
if map is NULL and in this case para_munmap does not look at its second
argument. However, this is rather subtle, so introduce a new label out_unmap
and jump there only if we really must unmap the file.