Invoking trick-ICG without any arguments results in a Segmentation fault

ICG dies on a strdup if the the input file is empty.  Added a check for
an empty input file name.  We exit with an error message if there is
no input file given.

refs #229

trick_source/codegen/Interface_Code_Gen/main.cpp

@@ -88,6 +88,11 @@ int main( int argc , char * argv[] ) {
         return 0 ;
     }
 
+    if ( input_file_names.empty() ) {
+        std::cerr << "No header file specified" << std::endl ;
+        return 1 ;
+    }
+
     ci.createDiagnostics();
     clang::DiagnosticOptions & diago = ci.getDiagnosticOpts() ;
     diago.ShowColors = 1 ;