Avoid integer cast issues when deserializing tags

`buffer` contains signed chars, so if a tag length is greater than 128 then it
is treated as a negative value when deserializing. The negative signed char is
then implicitly cast to a large unsigned integer. Explicitly cast the values to
signed chars

Co-authored-by: Rahul Zhade <zhade3@github.com>

src/scanner.cc

@@ -60,7 +60,7 @@ struct Scanner {
         Tag &tag = tags[j];
         tag.type = static_cast<TagType>(buffer[i++]);
         if (tag.type == CUSTOM) {
-          unsigned name_length = buffer[i++];
+          unsigned name_length = (unsigned char)buffer[i++];
           tag.custom_tag_name.assign(&buffer[i], &buffer[i + name_length]);
           i += name_length;
         }