Fix two unchecked mallocs in crypto/asn1.

Imported from upstream's 918bb8652969fd53f0c390c1cd909265ed502c7e. (The
remainder is in code we've since deleted.)

Change-Id: Ie878272114086ba60a0fd5eae059b641b00ec0c4
Reviewed-on: https://boringssl-review.googlesource.com/3831
Reviewed-by: Adam Langley <agl@google.com>