Fix memory leak on malloc failure.

Found by running malloc tests with -valgrind. Unfortunately, the next one is
deep in crypto/asn1 itself, so I'm going to stop here for now.

Change-Id: I7a33971ee07c6b7b7a98715f2f18e0f29380c0a1
Reviewed-on: https://boringssl-review.googlesource.com/3350
Reviewed-by: Adam Langley <agl@google.com>