external/boringssl: Sync to 3a18bf04745c0ef9850efeec1a52e402c4392388.

This includes the following changes:

https://boringssl.googlesource.com/boringssl/+log/9a127b43b8b78a135d6b64a3e25b8a704c2c069f..3a18bf04745c0ef9850efeec1a52e402c4392388

Test: BoringSSL CTS Presubmits.
Change-Id: Id39a146b1a6d7483f4142a7c191796ba65f32883
diff --git a/src/crypto/fipsmodule/bn/mul.c b/src/crypto/fipsmodule/bn/mul.c
index 7cc0e3c..a4e27f2 100644
--- a/src/crypto/fipsmodule/bn/mul.c
+++ b/src/crypto/fipsmodule/bn/mul.c
@@ -230,11 +230,13 @@
             if (--dl <= 0) {
               break;
             }
+            OPENSSL_FALLTHROUGH;
           case 2:
             r[2] = a[2];
             if (--dl <= 0) {
               break;
             }
+            OPENSSL_FALLTHROUGH;
           case 3:
             r[3] = a[3];
             if (--dl <= 0) {