Fix conversion to hex of negative characters

Negative characters were previously converted with unneccessary leading
`f` characters. For example `-1` or `0xff` characters were previously
printed as `_ffffffff`. This commit uses the value of characters in the
-128 to -1 range remapped into the 128 to 255 range, so that `-1` prints
as `_ff`, without unneccessary padding with leading `f`s.

Author: thomasspriggs

src/util/string_utils.cpp

@@ -163,8 +163,7 @@ std::string escape_non_alnum(const std::string &to_escape)
     else if(isalnum(uch))
       escaped << ch;
     else
-      escaped << '_' << std::hex << std::setfill('0') << std::setw(2)
-              << (unsigned int)ch;
+      escaped << '_' << std::hex << std::setfill('0') << std::setw(2) << uch;
   }
   return escaped.str();
 }