Fix two bugs found by a new clang warning I'm currently testing:

base/md5.cc

 // The original file was copied from sqlite, and was in the public domain.
 // Modifications Copyright 2006 Google Inc. All Rights Reserved
 
 /*
  * This code implements the MD5 message-digest algorithm.
  * The algorithm is due to Ron Rivest.  This code was
  * written by Colin Plumb in 1993, no copyright is claimed.
  * This code is in the public domain; do with it what you wish.
  *
  * Equivalent code is available from RSA Data Security, Inc.
@@ skipping 229 matching lines @@
         }
         byteReverse(ctx->in, 14);
 
         /* Append length in bits and transform */
         ((uint32 *)ctx->in)[ 14 ] = ctx->bits[0];
         ((uint32 *)ctx->in)[ 15 ] = ctx->bits[1];
 
         MD5Transform(ctx->buf, (uint32 *)ctx->in);
         byteReverse((unsigned char *)ctx->buf, 4);
         memcpy(digest->a, ctx->buf, 16);
-        memset(ctx, 0, sizeof(ctx));    /* In case it's sensitive */
+        memset(ctx, 0, sizeof(*ctx));    /* In case it's sensitive */
 }
 
 std::string MD5DigestToBase16(const MD5Digest& digest){
   static char const zEncode[] = "0123456789abcdef";
 
   std::string ret;
   ret.resize(32);
 
   int j = 0;
   for (int i = 0; i < 16; i ++) {
     int a = digest.a[i];
     ret[j++] = zEncode[(a>>4)&0xf];
     ret[j++] = zEncode[a & 0xf];
   }
   return ret;
 }
 
 void MD5Sum(const void* data, size_t length, MD5Digest* digest) {
   MD5Context ctx;
   MD5Init(&ctx);
   MD5Update(&ctx, static_cast<const unsigned char*>(data), length);
   MD5Final(digest, &ctx);
 }
 
 std::string MD5String(const std::string& str) {
   MD5Digest digest;
   MD5Sum(str.data(), str.length(), &digest);
   return MD5DigestToBase16(digest);
 }