Merge in upstream's certificate corpus.

This was done by running:

  ./fuzz/cert -merge=1 ../fuzz/cert_corpus ~/openssl/fuzz/corpora/x509

I bumped the max_len while doing so because some of those are rather
large.

Change-Id: Ic2caa09d5ff9ab05b46363940a91a03f270cbad8
Reviewed-on: https://boringssl-review.googlesource.com/12682
Reviewed-by: Adam Langley <agl@google.com>

FUZZING.md

@@ -23,7 +23,7 @@ Then copy `libFuzzer.a` to the top-level of your BoringSSL source directory.
 From the `build/` directory, you can then run the fuzzers. For example:
 
 ```
-./fuzz/cert -max_len=3072 -jobs=32 -workers=32 ../fuzz/cert_corpus/
+./fuzz/cert -max_len=10000 -jobs=32 -workers=32 ../fuzz/cert_corpus/
 ```
 
 The arguments to `jobs` and `workers` should be the number of cores that you wish to dedicate to fuzzing. By default, libFuzzer uses the largest test in the corpus (or 64 if empty) as the maximum test case length. The `max_len` argument overrides this.
@@ -32,7 +32,7 @@ The recommended values of `max_len` for each test are:
 
 | Test          | `max_len` value |
 |---------------|-----------------|
-| `cert`        | 3072            |
+| `cert`        | 10000           |
 | `client`      | 20000           |
 | `pkcs8`       | 2048            |
 | `privkey`     | 2048            |