Check for empty basename in GetFileContentOrDie() (#1558)

An empty basename would mean a directory path is returned by
GetFullPathOrDie(), which then leads to a cryptic exception when trying
to read from it using the istreambuf_iterator:

"basic_filebuf::underflow error reading the file: iostream error"

cartographer/common/configuration_file_resolver.cc

@@ -47,6 +47,7 @@ std::string ConfigurationFileResolver::GetFullPathOrDie(
 
 std::string ConfigurationFileResolver::GetFileContentOrDie(
     const std::string& basename) {
+  CHECK(!basename.empty()) << "File basename cannot be empty." << basename;
   const std::string filename = GetFullPathOrDie(basename);
   std::ifstream stream(filename.c_str());
   return std::string((std::istreambuf_iterator<char>(stream)),