patch 9.0.0820: memory leak with empty shell command

Problem:    Memory leak with empty shell command.
Solution:   Free the empty string.

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    820,
 /**/
     819,
 /**/