Return syntax error message when issuing invalid instance specific control...

Return syntax error message when issuing invalid instance specific control command in VLM. ("control test thisisaninstanceid play" is valid, "control test thisisaninstanceid asdqsd" isn't).

src/misc/vlm.c

@@ -619,6 +619,12 @@ static int ExecuteCommand( vlm_t *p_vlm, const char *psz_command,
                 psz_instance = ppsz_command[2];
 
                 if( i_command < 4 ) goto syntax_error;
+
+                if( strcmp( ppsz_command[3], "play" ) &&
+                    strcmp( ppsz_command[3], "stop" ) &&
+                    strcmp( ppsz_command[3], "pause" ) &&
+                    strcmp( ppsz_command[3], "seek" ) )
+                    goto syntax_error;
             }
 
             psz_command = ppsz_command[i_index];