patch 8.2.5126: substitute may overrun destination buffer

Problem:    Substitute may overrun destination buffer.
Solution:   Disallow switching buffers in a substitute expression.

src/ex_docmd.c

@@ -7138,9 +7138,10 @@ do_exedit(
 #endif
 	    )
     {
-	// Can't edit another file when "curbuf_lock" is set.  Only ":edit"
-	// can bring us here, others are stopped earlier.
-	if (*eap->arg != NUL && curbuf_locked())
+	// Can't edit another file when "textlock" or "curbuf_lock" is set.
+	// Only ":edit" or ":script" can bring us here, others are stopped
+	// earlier.
+	if (*eap->arg != NUL && text_or_buf_locked())
 	    return;
 
 	n = readonlymode;

src/testdir/test_substitute.vim

@@ -1063,6 +1063,19 @@ func Test_sub_open_cmdline_win()
   call delete('Xresult')
 endfunc
 
+" This was editing a script file from the expression
+func Test_sub_edit_scriptfile()
+  new
+  norm o0000000000000000000000000000000000000000000000000000
+  func EditScript()
+    silent! scr! Xfile
+  endfunc
+  s/\%')/\=EditScript()
+
+  delfunc EditScript
+  bwipe!
+endfunc
+
 " Test for the 2-letter and 3-letter :substitute commands
 func Test_substitute_short_cmd()
   new

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    5126,
 /**/
     5125,
 /**/