md5: Return a NULL pointer if MD5::raw_digest() is called early

This never happens since MD5::raw_digest()'s only caller, util::md5(),
always does it right. However, it's still an open invitation for someone
to read garbage past the end of a 1 byte array, or even worse, write
into it. I'd rather have us crash by dereferencing the null page in that
case instead.

src/md5.cpp

@@ -163,7 +163,7 @@ MD5::uint1 *MD5::raw_digest()
   if (!finalized){
     std::cerr << "MD5::raw_digest:  Can't get digest if you haven't "<<
       "finalized the digest!" <<std::endl;
-    return ( const_cast<uint1*>(reinterpret_cast<const uint1*>("")));
+    return NULL;
   }
 
   memcpy(s, digest, 16);