patch 9.0.0820: memory leak with empty shell command

Problem:    Memory leak with empty shell command.
Solution:   Free the empty string.
diff --git a/src/version.c b/src/version.c
index f9a00d6..2d2ae6a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    820,
+/**/
     819,
 /**/
     818,