Fix leak with ASN.1 combine.

When parsing a combined structure pass a flag to the decode routine
so on error a pointer to the parent structure is not zeroed as
this will leak any additional components in the parent.

This can leak memory in any application parsing PKCS#7 or CMS structures.

CVE-2015-3195.

Thanks to Adam Langley (Google/BoringSSL) for discovering this bug using
libFuzzer.

PR#4131

Reviewed-by: Richard Levitte <levitte@openssl.org>