patch 8.2.4076: memory leak in autoload import

Problem:    Memory leak in autoload import.
Solution:   Do not overwrite the autoload prefix.

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4076,
 /**/
     4075,
 /**/

src/vim9script.c

@@ -133,7 +133,8 @@ ex_vim9script(exarg_T *eap UNUSED)
     si->sn_state = SN_STATE_HAD_COMMAND;
 
     // Store the prefix with the script.  It isused to find exported functions.
-    si->sn_autoload_prefix = get_autoload_prefix(si);
+    if (si->sn_autoload_prefix == NULL)
+	si->sn_autoload_prefix = get_autoload_prefix(si);
 
     current_sctx.sc_version = SCRIPT_VERSION_VIM9;
     si->sn_version = SCRIPT_VERSION_VIM9;