Don't access invalid memory. (#2986)

* Don't access invalid memory.

* Remove unnecessary cast.

pdal/util/Utils.cpp

@@ -436,8 +436,8 @@ std::string Utils::escapeJSON(const std::string &str)
     };
     for (std::string::size_type i = 0; i < s.size();)
     {
-        char val = s[i];
-        if (val < (char)replacements.size())
+        unsigned char val = s[i];
+        if (val < replacements.size())
         {
             s.replace(i, 1, replacements[val]);
             i += replacements[val].size();