Fix error in ConCatPathFileName (#4575)

include/flatbuffers/stl_emulation.h

@@ -38,6 +38,10 @@ namespace flatbuffers {
 
 // Retrieve ::back() from a string in a way that is compatible with pre C++11
 // STLs (e.g stlport).
+inline char& string_back(std::string &value) {
+  return value[value.length() - 1];
+}
+
 inline char string_back(const std::string &value) {
   return value[value.length() - 1];
 }

include/flatbuffers/util.h

@@ -216,7 +216,7 @@ inline std::string ConCatPathFileName(const std::string &path,
                                       const std::string &filename) {
   std::string filepath = path;
   if (filepath.length()) {
-    char filepath_last_character = string_back(filepath);
+    char &filepath_last_character = string_back(filepath);
     if (filepath_last_character == kPathSeparatorWindows) {
       filepath_last_character = kPathSeparator;
     } else if (filepath_last_character != kPathSeparator) {