ISclose() doesn't free memory if a stream's function pointer, close, is NULL.
However, str_stream's close pointer is always NULL and thus leaks
memory.
Take the correct type from ASN1_STRING_get0_data() to fix a warning.
IA5Strings are ASCII, we do not need to care about signedness.
Since the pointer returned by ASN1_STRING_get0_data() points to internal
memory of the ASN1_STRING we are not allowed to manipulate the data.
Also it is not guaranteed that the data is null terminated. Copy the
data to our own buffer and make sure it is null terminated before using
it.
