vim-patch:9.0.1145: invalid memory access with recursive substitute expression (#23132)

Problem:    Invalid memory access with recursive substitute expression.
Solution:   Check the return value of vim_regsub().

3ac1d97a1d

Co-authored-by: Bram Moolenaar <Bram@vim.org>

test/old/testdir/test_substitute.vim

@@ -1109,6 +1109,22 @@ func Test_sub_expr_goto_other_file()
   bwipe!
 endfunc
 
+func Test_recursive_expr_substitute()
+  " this was reading invalid memory
+  let lines =<< trim END
+      func Repl(g, n)
+        s
+        r%:s000
+      endfunc
+      next 0
+      let caught = 0
+      s/\%')/\=Repl(0, 0)
+      qall!
+  END
+  call writefile(lines, 'XexprSubst', 'D')
+  call RunVim([], [], '--clean -S XexprSubst')
+endfunc
+
 " Test for the 2-letter and 3-letter :substitute commands
 func Test_substitute_short_cmd()
   new