Don’t assume first CL arg (exe name) is present

Fixes #729

include/external/clara.h

@@ -927,7 +927,7 @@ namespace Clara {
         }
 
         std::vector<Parser::Token> parseInto( std::vector<std::string> const& args, ConfigT& config ) const {
-            std::string processName = args[0];
+            std::string processName = args.empty() ? std::string() : args[0];
             std::size_t lastSlash = processName.find_last_of( "/\\" );
             if( lastSlash != std::string::npos )
                 processName = processName.substr( lastSlash+1 );