Make MSVC happy.

The MSVC build is failing with:
  ssl\s3_srvr.c(1363) : warning C4701: potentially uninitialized local variable 'digest_len' used

I don't believe that this warning is valid, but this change assigns a
value to |digest_len| to fix the build.

Change-Id: I20107a932bc16c880032cc1a57479b1a806aa8ea
Reviewed-on: https://boringssl-review.googlesource.com/6821
Reviewed-by: Adam Langley <agl@google.com>

ssl/s3_srvr.c

diff --git a/ssl/s3_srvr.c b/ssl/s3_srvr.c
index 8ca2cf9..72cd1c4 100644
--- a/ssl/s3_srvr.c
+++ b/ssl/s3_srvr.c
@@ -1344,7 +1344,7 @@
 
       /* Compute the digest and sign it. */
       uint8_t digest[EVP_MAX_MD_SIZE];
-      unsigned digest_len;
+      unsigned digest_len = 0;
       EVP_MD_CTX md_ctx;
       EVP_MD_CTX_init(&md_ctx);
       int digest_ret =