don't add integers to string literals (#410)

* string literal + integer means unintended and incorrect pointer arithmetic

fixes a clang warning. it could not be triggered, because it can only be
triggered if the string given to getopt is not covered among the
cases in the switch.

* handle review comment

benchmark/parse.cpp

@@ -126,7 +126,10 @@ struct option_struct {
           }
           break;
         default:
-          exit_error("Unexpected argument " + c);
+          // reaching here means an argument was given to getopt() which did not have a case label
+          exit_error("Unexpected argument - missing case for option "+
+                     std::string(1,static_cast<char>(c))+
+                     " (programming error)");
         }
       }
     #else