fixed 64 bit to 32 bit variable size mismatch warning in base64 encoder

src/base64/base64.cpp

@@ -82,7 +82,7 @@ std::string base64_encode(unsigned char const* bytes_to_encode, unsigned int in_
 }
 
 std::string base64_decode(std::string const& encoded_string) {
-  int in_len = encoded_string.size();
+  size_t in_len = encoded_string.size();
   int i = 0;
   int j = 0;
   int in_ = 0;