Fix and/or annotate all switch fall-throughs.

In some configurations, Clang will warn about all unannotated
fall-throughs in C++. This change adds the needed annotation for Clang
in the single place where we appear to have this.

Change-Id: I25a9069e659ce278d3cd24bf46f667324b3d5146
Reviewed-on: https://boringssl-review.googlesource.com/18024
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

ssl/ssl_cert.cc

diff --git a/ssl/ssl_cert.cc b/ssl/ssl_cert.cc
index df4b9c8..a410aef 100644
--- a/ssl/ssl_cert.cc
+++ b/ssl/ssl_cert.cc
@@ -593,6 +593,7 @@
       break;
     case -2:
       OPENSSL_PUT_ERROR(X509, X509_R_UNKNOWN_KEY_TYPE);
+      break;
     default:
       assert(0);
       break;