Fix a regression in 0.17 that broke displaying graphics over SSH

The new types GraphicsCommand can generate a command with only a payload
and no control keys, as the last command during chunked transfer.
Fix the parser to handle this. Fixes #2568

gen-apc-parsers.py

@@ -137,6 +137,7 @@ static inline void
     {parr}
     {keys_enum}
     enum KEYS key = '{initial_key}';
+    if (screen->parser_buf[pos] == ';') state = AFTER_VALUE;
 
     while (pos < screen->parser_buf_pos) {{
         switch(state) {{