Fixed crash related to flatc parsing duplicate input files.

Thanks @Chaosvex for reporting.

Change-Id: I73f60ab0bf875a3e0849eaec5f42f6d036881094
Tested: on Linux.

include/flatbuffers/idl.h

@@ -419,7 +419,7 @@ class Parser {
   const char *source_, *cursor_;
   int line_;  // the current line being parsed
   int token_;
-  std::stack<std::string> files_being_parsed_;
+  std::string files_being_parsed_;
   bool proto_mode_;
   bool strict_json_;
   std::string attribute_;

src/idl_parser.cpp

@@ -892,7 +892,7 @@ EnumDef &Parser::ParseEnum(bool is_union) {
   Expect(kTokenIdentifier);
   auto &enum_def = *new EnumDef();
   enum_def.name = enum_name;
-  if (!files_being_parsed_.empty()) enum_def.file = files_being_parsed_.top();
+  enum_def.file = files_being_parsed_;
   enum_def.doc_comment = enum_comment;
   enum_def.is_union = is_union;
   enum_def.defined_namespace = namespaces_.back();
@@ -973,7 +973,7 @@ StructDef &Parser::StartStruct(const std::string &name) {
   if (!struct_def.predecl) Error("datatype already exists: " + name);
   struct_def.predecl = false;
   struct_def.name = name;
-  if (!files_being_parsed_.empty()) struct_def.file = files_being_parsed_.top();
+  struct_def.file = files_being_parsed_;
   // Move this struct to the back of the vector just in case it was predeclared,
   // to preserve declaration order.
   *remove(structs_.vec.begin(), structs_.vec.end(), &struct_def) = &struct_def;
@@ -1352,11 +1352,11 @@ Type Parser::ParseTypeFromProtoType() {
 
 bool Parser::Parse(const char *source, const char **include_paths,
                    const char *source_filename) {
+  files_being_parsed_ = source_filename ? source_filename : "";
   if (source_filename &&
       included_files_.find(source_filename) == included_files_.end()) {
     included_files_[source_filename] = true;
     files_included_per_file_[source_filename] = std::set<std::string>();
-    files_being_parsed_.push(source_filename);
   }
   if (!include_paths) {
     static const char *current_directory[] = { "", nullptr };
@@ -1499,10 +1499,8 @@ bool Parser::Parse(const char *source, const char **include_paths,
       error_ += NumToString(line_) + ":0";  // gcc alike
     #endif
     error_ += ": error: " + msg;
-    if (source_filename) files_being_parsed_.pop();
     return false;
   }
-  if (source_filename) files_being_parsed_.pop();
   return true;
 }