Avoid emitting empty string at end of $display translation

This happened if the last part of a format string was a % code.

tgt-vhdl/display.cc

@@ -170,6 +170,8 @@ int draw_stask_display(vhdl_procedural *proc, stmt_container *container,
                ss << *p;
          }
 
+         // Call Write on any non-empty string data left in the buffer
+         if (!ss.str().empty())  
             display_write(container, new vhdl_const_string(ss.str().c_str()));
       }
       else {