patch 8.2.5169: nested :source may use NULL pointer

Problem:    Nested :source may use NULL pointer.
Solution:   Do not use the NULL pointer.

src/eval.c

@@ -2387,12 +2387,15 @@ eval0_retarg(
 
     p = skipwhite(arg);
     ret = eval1(&p, rettv, evalarg);
+
+    if (ret != FAIL)
+    {
 	expr_end = p;
 	p = skipwhite(p);
 
 	// In Vim9 script a command block is not split at NL characters for
-    // commands using an expression argument.  Skip over a '#' comment to check
-    // for a following NL.  Require white space before the '#'.
+	// commands using an expression argument.  Skip over a '#' comment to
+	// check for a following NL.  Require white space before the '#'.
 	if (in_vim9script() && p > expr_end && retarg == NULL)
 	    while (*p == '#')
 	    {
@@ -2406,8 +2409,10 @@ eval0_retarg(
 		check_for_end = FALSE;
 	    }
 
-    if (ret != FAIL && check_for_end)
+	if (check_for_end)
 	    end_error = !ends_excmd2(arg, p);
+    }
+
     if (ret == FAIL || end_error)
     {
 	if (ret != FAIL)
@@ -2433,7 +2438,8 @@ eval0_retarg(
 	// Some of the expression may not have been consumed.  Do not check for
 	// a next command to avoid more errors, unless "|" is following, which
 	// could only be a command separator.
-	if (eap != NULL && skipwhite(p)[0] == '|' && skipwhite(p)[1] != '|')
+	if (eap != NULL && p != NULL
+			  &&  skipwhite(p)[0] == '|' && skipwhite(p)[1] != '|')
 	    eap->nextcmd = check_nextcmd(p);
 	return FAIL;
     }

src/testdir/test_vimscript.vim

@@ -7528,6 +7528,25 @@ func Test_for_over_string()
   call assert_equal('', res)
 endfunc
 
+" Test for deeply nested :source command  {{{1
+func Test_deeply_nested_source()
+  let lines =<< trim END
+
+      so
+      sil 0scr
+      delete
+      so
+      0
+  END
+  call writefile(["vim9 silent! @0 \n/"] + lines, 'Xnested.vim')
+
+  " this must not crash
+  let cmd = GetVimCommand() .. " -e -s -S Xnested.vim -c qa!"
+  call system(cmd)
+
+  call delete('Xnested.vim')
+endfunc
+
 "-------------------------------------------------------------------------------
 " Modelines								    {{{1
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker

src/version.c

@@ -735,6 +735,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    5169,
 /**/
     5168,
 /**/